In the constructor of TBCButtonFocus is this line:
Maybe that overrides the lfm saved property.
Try removing that line and rebuild lazarus. If it works for you I will change the sourcecode.
Yes, that seems to have fixed it, although there is no...
...line written to the lfm file to signify the False property state (the TabStop := True; line is written), but at least when you close/reopen Laz the TabStop property retains it's saved state.