I found that if the following fields were not value 0:
Cruise fuel burn
Climb fuel burn
Descent fuel burn
Holding fuel burn
Then the flight would not save. By setting exactly all four to 0, the flight would save. No value of cruise fuel burn (tried 0.1 through 150) except 0 would work.
I then checked a few aircraft selections which have performance data associated with them… couldn’t save. I assume they have non-zero values “baked in” to their fuel consumption which causes the same problem as my non-performance profiles with a manual non-zero fuel consumption figure.