Trials of Osiris titan mark was gone from vault after patch 2.0 (not dismantle) . Please check, and restore it.
And yes, i know about https://www.bungie.net/en/help/article/12211, but its different.
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First can you please do the following. 1. Check all characters inventory 2. Check the vaults 3. Log it of destiny and restart your console 4. Log back into destiny then do steps 1&2 If you still can't find your gear then there is no way to get it back sadly. No matter how it was lost, it will not be restored by Bungie.