I've received this feedback from the support desk about this behavior:
I have reviewed this ticket and actually this is an expected behavior in JIRA 5.0 and above. The reason behind this is to improve the performance by limiting the search using text suggestion. Returning all user with assignable user permission can cause a performance issue especially when the Instance having a large user base.
Unfortunately, you can not revert back the changes without modifying the source code which is not supported by Atlassian. I hope this clarifies your inquiry and please let us know if you have further question regarding this.
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