Hi @AndrewBowne,
Based on the info you shared, it seems you planned your mission correctly, and that the images were captured as expected.
I assume that some images did not calibrate during processing causing to the gap, and subsequent images to calibrate but incorrectly (North West corner). Without more details I can't confirm or explain why.
Please open a support ticket to get an analyst looking at this as soon as possible and resolved.
While Support reviews this, since you mentioned you were using an M300RTK, you can try reprocessing the mission but set the geolocation accuracy to 0.1 ft/ 0.02 m for XY and Z in the advanced processing settings if RTK was being used. If RTK was not in use, set it to 1 ft / 0.3 m XY & Z for testing.
You can set the output resolutions to the lowest settings for faster results to see if they improve, if they do, reuse the same geolocation accuracy setting but with the desired output resolution.
Regards,
Nico