Try another test, put everything on manual and we can build from there. Put the camera in manual, 1/125 f5.6 iso 200 with auto ISO off. Put the flash into manual and set it to something like 1/8th power. Try your run of shots again and review them, they should all be the same. If they aren’t then there’s a problem.
If they are the same, start introducing auto features, the first I would try is TTL on the flash and try again, put keeping the manual settings in the camera. If these are fine put the flash back into manual and your camera into an auto exposure mode (assuming that’s what you were using) and try again.
By rolling it back to basics and using auto features only in one place at once you should be able to determine which part exactly is playing up
If you still need help get some of the shots uploaded with the EXIF in tact and post them in here and we can see if we can help any further