Yes, totally normal and the narrower the band pass of the filter, the more band pass sift one will typically encounter.
Not everyone is good about publishing the maximum focal ratio of the filters, but some do. In the 5nm to 6nm range, band shift will start off axis at about f/2.8. Baader has "High Speed" filters that they say are good to f/2.
When running at 1x, you are not really using a lens or objective, so here it is just the angle of incidence for the subject. something at the center of the field would be 0 degrees, and something at the outside of the field would be 20 degrees. Even using a 6nm filter Astronomic, I cannot get the entire Barnards's Loop into the narrow band portion of the field at the same time so I have to pan around the nebula to see it well and only the part that is near the center of the field will be shown at full brightness and unless I put it in the center of the field, I can't see the Anglefish nebula.
Astronomic says this about their 6nm (which I take to mean f/3.75 to f/15)
The design of the transmission curve makes it possible to use the filters with instruments from 1:3.75 to 1:15
This would mean that only the light coming in from the angle of an f/3.75 light cone (or about the central 30% of the field) would be fully in band.
The Astrodon 3nm is rated down to f/3 so with the 1x lens, only the central 1/3rd of the field is in band.
3 nm filters can be used on systems to f/3
Under dark skies with my binocular, the 12nm filters actually gave the best view of Barnard's Loop because while the pass was not narrow, under dark skies it is not super critical, and with the 12s, I can see the entire nebula in the field at once without having to pan.
Now not everyone provides a maximum focal ratio for the field to be in band, but if they do, then you can pretty easily determine about how much of the field will be in band in your application.
I use a 5nm rated for f/3 in my Boren Simon, and in this scope, almost all of the field is in band, making it an excellent scope for fields where the nebula is larger than the true field of the scope. If there is band sift, it can look like the nebula does not fully cover the field and when you are under dark skies working along the Milky Way, there are many areas where the entire 2.4 degree true field of the Boren Simon shows nebula even if no catalog items are present. Big parts of the Milky Way are simply carpeted over with nebula, but if you use a very fast lens that is too fast for the filter, you only see the nebula at the center of the field and reach the conclusion that this is all the nebula that there is, especially if you are combining that band sift with any amount of vignetting. Another reason why I like f/2.8. It is about as fast as I can go before band-shift and vignetting start to obscure too much nebula out there at the edge of the field. There is more noise at f/2.8 than f/1.2, but I would rather have the full aperture of the filter (or close to it) in band than have a big area of it out of band and vignetted.