I stumbled across this odd behavior.  I'm not really sure if this is a bug or an idiosyncrasy of the way the site works.  I had a member that left, then rejoined 3 years later with a new account, but the same email address.  (I can only guess that they deleted their account in between since it has a different member ID, but it may also be because of changes to the site while they were gone, 7/16-9/19.)  If I find the original account in the Activity log and select one of their messages, it takes me to it.  If I then select More > All Messages By This Member, it gives me a list of many messages in the group, though none from that member.  In addition, the upper right says 1-50 of 10000, though there are 35428 messages on the group.  For another member that hasn't rejoined, the More > All Messages By This Member does show me all their messages correctly.  Probably not something that's going to happen often, so maybe not worth spending much time on.

Thanks,
Duane