OK, what's "short" and what's "longer" ? I take flats depending on filter at exposures from 0.01" to 1.2". We'll stipulate that 0.01" is short, but is 1.2" "longer"?
Also I keep hearing this about bias, but I have an ASI1600MM-C, which is a Panasonic "M" camera, correct? I have been using a superbias (100 bias frames) taken at shortest possible exposure with SGP for *years* now, with no ill effects (following the Warren Keller approach in Inside Pixinsight). I am tempted to take some dark flats just to compare and see if there's any discernible difference, but it's not going to be major for sure.
Fraction of a second is definitely "shorter." With these CMOS cameras, usually there are two types of exposure control. Camera controlled and driver controlled. Now at one point the cutoff was different for ZWO cameras, but I think these days both QHY and ZWO use 1 second as the cutoff point for camera-controlled exposures. Below, the camera controls the exposure (hardware timing is necessary to get exact sub-second exposure lengths, basically). Above 1 second, the driver controls exposure. Now right around 1 second or so, the actual time the sensor is exposed before readout may not be exactly 1 second, since there can be some latency between the driver issuing the command to end exposure, and the camera actually receiving it and ending exposure. Once you get up to a few seconds (what I would call "longer"), then this discrepancy doesn't really matter since it is usually just a fraction of a second anyway.
As for bias. The Panasonic M-series sensors SPECIFICALLY (and only them) have some issues with extremely short exposures. Under 0.2 seconds, there is an inconsistent signal that will fill the frame...but, it does not fill it the same each frame. So literal bias frames, 0 second exposures (the minimum exposure on the Panasonic M cameras is usually 32 microseconds, or 32 thousandths of a millisecond), will usually exhibit a changing gradient across the frame from one frame to the next. This variable gradient tends to result in a sort of radial gradient once all the frames are integrated, with a darkish spot in one area somewhere within the frame that brightens to the edges. The crazy thing is, this gradient where it gets brightest can actually be up to a few ADU brighter than the configured bias offset level itself. As you start to increase exposure from 0 to 0.2 seconds, the level of the bias will start to increase. So there is also inconsistency in the mean level of the biases for exposures below 0.2 seconds:
When you use 0.2 second or longer exposures, this odd variability in the frames disappears. The frames become consistent, totally flat, and the entire frame falls within the read noise RMS of the configured bias offset. You can see in the chart above how mean and median levels flatten out once you reach 0.2 seconds. From that point on, you will slowly start to see an increase in mean level due to dark current, but it is nothing like when exposures are under 0.2 seconds.
Note that this testing was done on an ASI1600MM-Cool, and done some time ago. The same kind of testing was also actually done by a couple other people, one guy actually did similar tests before I did and at the time I wasn't sure I believed his results, then I did my own testing and found the same problem. There have been tests with both QHY and ZWO cameras, and both manufacturers have recommended 0.3 second minimum exposures (I think QHY officially documented it in their camera manual, I don't recall if ZWO did, but I know ZWO recommends it on their forums).
NOW, I am not sure if these manufacturers recommend 0.3 second exposures for ALL CMOS cameras. If they do, it may just be a blanket precautionary thing. In my own testing and real-world experience with the ASI183, as well as my experience assisting at least a couple dozen other people with calibration struggles with the IMX183 cameras in general, I can say with solid confidence that the IMX183 does NOT have the same problem. It's exposures work well all the way down to minimum exposure ("zero seconds" with most acquisition programs). You can use bias frames for calibrating short flats if you wish with the IMX183, there is no specific short exposure limitation. You can use flats down to tiny fractions of a second if you need to.
The Panasonic M cameras are fine cameras...but they do need a little bit more care and attention to the calibration frames because of the bias instability issue. Just keep in mind, the Panasonic M issues are just for that sensor, not every CMOS sensor on the market. Also keep in mind, the Panasonic M sensor was really originally designed as a M4/3 mirrorless camera/video sensor, and was generally intended to be used for video capture with an Olympus camera. So it wasn't explicitly designed for our kind of ultra low light market. It is sadly the only M4/3 sized mono CMOS sensor on the market right now, so if you need fast downloads, or need high gain for short NB exposures, or anything like that...there really are no alternatives. The IMX183 sensor is just slightly over 1/2 the area of the Panasonic M sensor...HALF the area! ;P
I am hopeful that there will be a small pixel mono APS-C sensor on the market, preferably from Sony (as they really do seem to know how to make a good sensor), sometime soon here. In all the talk about the forthcoming IMX455 (full frame, 36x24mm, mono CMOS), I think there were a couple of posts about a mono Sony APS-C that might find its way into a QHY camera at some point here. So there is hope that we'll get a larger mono sensor some time within the next year here, for those who need a larger frame and don't want to deal with the calibration nuances of the Panasonic M.