I know I have seen you post about technical issues before, and my guess is you are knowledgeable about this site.
I read your phrase:
And without warning my PM box dropped to 50 messages capacity,
as if you weren't expecting it to happen when your PA expired, so my bad if my reading comprehension failed me on that one.
Since only you know what you did, I can't offer any explanation to your issue. It seems beyond the realm of what most folks can explain.
I'll give you one example using mine. My private messages total at this moment is 68 messages. 45 messages are in my Inbox folder and 23 are in my Sent folder. If my Premium Access were to expire, my storage capacity would drop to 50 total. Since my capacity drops,
I expect the system to delete my excess over 50 to bring it down to the maximum of 50. If the programmer did things properly, I expect the system to delete the oldest messages from both folders, to get my Private messages within compliance.
It isn't written anywhere that I am aware of, but if your Max is 50 when your account reverts back to Valued Poster, and like in my case, you were over the maximum total, why should the Messaging system continue storing more than the maximum allowed for anyone?
Now, if you were under 50 messages when your PA expired and it still deleted your messages on its own, you'll need an expert to give you answers to that one. And if this is the case, I apologize for wasting your time. I was only trying to clarify things. And I know, sometimes we can make things worse despite any good intentions.
Good luck getting an answer to your question.