Gee, nothing "wrong"--you have a very nice image!
From the depth of my experience (I have made three, no four now ) I found a couple of things: camera rotation is not perfectly north-south or east-west, which is what the planner expects unless you set a specific angle. Your platesolved position can also be not as close as you would want for the second frame. In my first attempt, it missed an overlap completely!
My solution for the first problem is to be _very_ finicky about how I position the camera relative to the OTA. Easiest for me is to point my rig to the meridian in the south and then "square up" the camera alignment in whatever orientation I have chosen for my image (single frame or mosaic both). I am using NINA and am finding it a bit finicky when it comes to telling me my camera rotation angle when platesolving if I tell it I am using a manual rotator, so I turned that option off and just eyeball it. Orientation from session to session is usually within a couple of degrees, so I don't lose too much when combining images from multiple nights (certainly something that will be necessary for many if not most mosaic projects). I would like to figure out how to get NINA to play nice for rotation...
For the second issue: check what pointing tolerance your platesolving program is set for or can be adjusted to. Smaller is better--as I learned the hard way. Initially set at default of (as I recall) 2 arcminutes, I found advice here or elsewhere (NINA discord, I think) to lower that down to 0.2 arcmin. Platesolving takes another iteration or two, but works much better to align panels that way.
Now you can listen to the advice of the real experts out there, but without knowing more about your setup, those are the things I learned about similar results that have so far been working.
But as I said, you have a very nice image there. In fact, I kind of like the "ragged edge" look--makes everyone know it is a mosaic and you can puff out your chest and say, "this is my mosaic of the Seagull" and everyone can oooh and aaah and not wonder if you just used a big honking wide angle fast-optics mucho expensive setup