Do you have the same issue if you follow the OP’s steps to reproduce it?
The V-FP does not disarm after using it on approaches RNAV/RNP preventing it from performing another leg and requiring a restart of the aircraft in the menu. This is a bug that is present from August 2023.
I still have this problem as well.
Out of curiosity, when you lower the altitude preselect before top of climb, does the autopilot also start losing 500fpm even when altitude hold is on? I don’t think they are directly related, but its two major issues with vertical navigation with the autopilot that have both been around since release.