Welcome to Community! JSM provides 3 free agents and Jira provide max 10 users on free tier. these user count accumulates to the max number allowed. Therefore the reason you're facing this issue is most probably due to your Product Access for Jira Service Management being turned on, navigate to Site settings > Product access and turn off the default group for JSM. That way if you add any user, it should default to Jira (being Jira Core) if you do not have any other Jira Application available on the Instance.
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