There was a known bug regarding these kind of mentions would fail that you can find at JRA-45836.
Here is a description of the issue from the bug ticket:
e.g: firstname.lastname@example.org may be filtered out when searching for m if there are many more users with @mydomain.com email address and at least 10 users alphabetically before mike.
If you're using a JIRA server 7 version before 7.1.8, then it is likely that this bug is affecting you. An upgrade to JIRA Core 7.1.8 or higher will fix this issue. There are two workarounds listed:
Try with a different user value (if you tried with username, try display name, or last name). The last name value works particularly well as it is quite unique.
Typing the first few characters of a user's name and hitting the TAB button will autocomplete and allow for the user to be mentioned (until you refresh the page) e.g. John = Joh...TAB
If you are on a version of JIRA above 7.1.8 or a Cloud version let me know which version and we'll go from there.
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