Interesting…
Thanks!
We looked into this, and found that there’s a point where one of the x-axis gyroscopes very suddenly diverges from the other, after which they maintain a constant offset, and stabilisation stops working properly
It’s quite unexpected, particularly since it’s not at a mode change or anything, where it’s more feasible that a bug could occur in some parameter resetting or whatnot - this seems to have been completely random. The other sensors and the other axes of that gyroscope seem to be unaffected, and if it weren’t for the vehicle being quite deep underwater at the time we might suspect it was from a cosmic-ray-induced bit-flip or something.
@williangalvani mentioned he’ll bring this up with some of the other ArduPilot developers, in case they’ve got ideas about what might have gone wrong 
Bug or otherwise, doing a manual gyroscope calibration should revert the issue, although understandably that may be a little challenging while the vehicle is in the water and far from stable land.