The Acorn counter on the League page shows how many Acorns you have collected while a part of that Club during that season. This is why it appears as 0 right after joining.
(Leaving a club will return your season Apples to 0, even if you rejoin the same club later.)
Your lifetime Acorns are not affected at all. You can confirm this by exiting the League page, completing a level, and checking the amount shown on the Acorn icon.
(Leaving a club will return your season Apples to 0, even if you rejoin the same club later.)
Your lifetime Acorns are not affected at all. You can confirm this by exiting the League page, completing a level, and checking the amount shown on the Acorn icon.