This is controlled by an internal pre-validate SPEL trigger, which calls cr::require_assign(assignee, group, type) or chg::require_assign(assignee, group) depending on the object type. Issue works the same as chg.
The function is in the obfuscated file so you can't modify it there. What you can do is to override the function and implement it the way you want (or not implement it). You'll need to experiment to find out the correct return values.
I have a vague recollection of seeing an option in some older version's options manager but I couldn't find anything in r12.9 when I checked, so if it still exists I'd like to know.