x-imapbase timestamp field being rewritten

I'm having a problem with various users having repeated downloads of the 
messages in their mailboxes.  Most, but not all, are using Mac Mail.app, 
although I'm not sure if this makes a difference.

The problem occurs intermittently, although it does seem to appear in 
clusters, i.e. multiple users will be affected at the same time.

For some reason, the X-IMAP field (in particular the first number, the 
time stamp) is being rewritten to the current time instead of its 
original value.  This causes all the messages to have new UIDL values, 
and so the POP client thinks that these are new messages.  The field in 
question is:

X-IMAP: 1213219525 0000000002

Although if the data is not stored in a placeholder message, the header 
is named X-IMAPbase.

Does anyone have any idea why that number would be changing?
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