OK I'm looking at your forest now and see the following plots you can still expand to:
-Blue: 12.
-Green: 4.
-Purple: 16. (not including the one you're currently expanding to).
So... that totals out to 32, which is then still well above the 24 remaining expansions that should mark the point where 50 expansions have been reached. :P
So far then it's still possible that indeed the threshold lies at 50 completed expansions....
I hope others can provide data too, so we can perhaps pinpoint if that's the trigger or not...