After user changes Full Name the @mention does not work correctly

Same behavior that was Fixed in https://jira.atlassian.com/browse/CONF-28401 on version 5.5 but reoccurring in 5.6 confluence.

Reproduce:

• create user with Full name User One
• @mention user in confluence
• change users Full Name to anything else
• try to @mention anything else

Expected

users full name anything else will be found and mentioned

Actual

user not found
BUT ...

• you can still @mention User One
• on save the @mention changes to anything else

Additional Note:
Content re-index does not clear this out