I too though it may have to do with Gyro temp so I tried to do some tests insulating the flight controller (and then removing the insulation). There seems to be no correlation between IMU temp consistency and to the issue unfortunately.
It seems that longer and faster I fly, the greater the “-” variance value is about 60 sec after I land.
The graph below depicts the variance issue. What does NKF8 IYAW actually mean though? Does this have to do with different compass variations? Does it have to do with gyro variations?
The warning is given when the variance the value seems to go over -/+20deg.
Here is the log: https://www.dropbox.com/s/069qx245i3y4qbz/log_15_2020-5-9-18-27-18.bin?dl=1