Upon review of the MAgE (Migration Agent for Exchange) log there is the following informational line.
2015-02-27 21:26:45.7683 PxD20 TxF A4 C1 M1 nfo Ignoring Target MDB setting for the mailbox '/o=Company/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=John.Doe' from the calendar collection because this mailbox is added to a mail collection.
MAgE Calendar Sync behaviour with respect to moving mailboxes differs slightly from the legacy CSA agent.
Previously a mailbox was only moved once whilst it is brand new, from stub (DSA placeholder) database to CSA collection database. The legacy CSA verifies if the mailbox was created by any legacy agent (mail or previous calendar sync) and it performs creation and move only if it this is not the case.
The MAgE Calendar agent will move the mailbox to the specified Database until you add mailbox to any Mail Sync collection. To be precise, the MAgE Calendar agent will ignore the Mailbox Database setting even if mailbox was removed from mail collection after processing (the information about previously sync'd mailboxes is stored in the Migration Manager's SQL configuration database).
This behaviour is described on MAgE Calendar collection properties as follows:
"These settings will be applied only to those mailboxes that are not members of any mailbox synchronization collections”.