may I ask you if you are seeing the effect on a service desk portal or is this a plain Jira Core/Software site?
I found the following suggestion, it might match with your case.
If it is Jira Cloud/Software in Custom Field Configuration for the picker field a group might have been chosen which is the wrong one (too restrictive).
Members of Community might be able to assist more in detail if you could kindly confirm of the way you took (Service Desk / Core or Software).
ok, I see. From your description you already took the right path although the restriction to 10 entries is annoying, what I absolutetely can understand.
I found an article on how to put a group into the approval step as an alternative but if you want to explicitly select an Business Manager this won't probably not fulfill your needs.
I am not aware of any alternative solution at this point, I'm afraid.
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