Do you have the same issue if you follow the OP’s steps to reproduce it?
No
Provide extra information to complete the original description of the issue:
I’ve tried to replicate this issue several times from roughly the same area (off the Essex coast with landing at Heathrow, at the same altitude as OP’s). I tried VFR direct, and IFR high/low, and was able to change the starting altitude in the NAVLOG without issues, and without it reverting to something other than what I set. I tested with the A320 NEO. Not sure what other variables to try.
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