Capacity limits on per user access list?

I'm running into an issue where access is being automatically revoked for users to a particular node, and I can't find a reason for this. We use ACL to grant access to specific nodes upon checkout (via Rules). This has been working fine for a few years now. But suddenly we're getting a lot of reports from people who had access one day, and then next day they don't. I can verify they are no longer listed on the view access list for the node. I can't find anything in the logs or anywhere else to help me figure out what would have triggered them to be dropped from the list. But the one we are hearing about is our most popular item, and I am wondering if we are hitting some kind of capacity with ACL lists. Currently, there are 409 users with view access to this node. But based on purchase history, there should be a few thousand. So many have definitely been dropped.

Can anyone confirm a capacity limit, or point me in the right direction to troubleshoot this problem?

Thanks!