Online Mode to Cached Exchange Mode problem

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Outlook version
Outlook 365 64 bit
Email Account
Office 365 Exchange
When switching modes from Online Mode to Cached Exchange Mode on shared mailboxes with many messages (>30,000), a huge number of emails is sent to the Deleted Items folders.

Given: CEM creates an OST file on the local machine, and the size of that file depends on the duration that CEM. If it is set to e.g. 3 months (default) it's not that large. But since I set it to 2 years, it's quite large approx 50 GB. When I switched from Online Mode to CEM @ 2 years duration, over 67,000 emails were copied to the Deleted Items folders in less than one day. This happened on two separate occasions in two different shared mailboxes. These emails were simply duplicates of what still existed in the mailbox.

Hypothesis: When Exchange Server was creating the OST file to support Cached Exchange Mode, it cached the large number of emails somewhere and once the OST file was created, it deleted them, putting them in the shared mailbox Deleted Items folder.

Problem: the large Deleted Items folder contributed to the shared mailbox's OST file exceeding the 95 GB limit, which reduced functionality of the shared mailbox.

Please somebody explain if the hypothesis is correct, and if not, what the heck happened? cheers.
Yes, they were all messages that were in the shared mailbox. By emptying the Deleted Items folders, no information was lost. The two engineering groups never delete anything.
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