The has been answered numerous times in many threads It isn't instantaneous. You must wait a couple mins-couple hours for the counter to reset and realize you are at 50 posts.
It's in the TOU...first place you should look if you have a question like this
https://www.icmag.com/ic/showthread.php?t=280
16. PM System: A minimum post count of 50 is required before being able to send/receive private messages, as well as use a custom avatar. Those who attempt to circumvent the minimum post count by turbo posting, or making posts with no worthwhile contribution, will have their posts deleted and/or the post count of the turbo poster reset to 0 by admin as a warning. Repeated turbo posting will result in banning without warning. Due to previous abuses, all posts made within the Introduction Forum will not be included in member post count totals.
It's in the TOU....
https://www.icmag.com/ic/showthread.php?t=280
You should always check the TOU first....more often than not, it's got your answer.
They'd be as likely to read that as they are to read the TOU they are asked to read before checking the checkbox saying they read it. Even if they did read it they'd forget by 50 and come asking the same question again....