I don't have time to catch up on this thread right now but good job on the leap second issue.

CN flight recorders update the UTC offset typically around five minutes after GPS lock.

Short term work around is to power up more than five minutes before launch, use 4 second record interval (which should mask the problem if the offset change happens in flight).

This is preliminary info... but seems to fit all the observed problems so far.

Best regard,
Evan Ludeman for CNi