Permission Issue Writing to a List Field

[fcaruso123]

We built a script that lets users link changes to other changes. Selecting a change to relate runs a business rule that adds the change record to a custom field.  It utilizes a List type field that is hidden. There is a related tab where users can related a change (Edit button) or create a new related change (New button):

Clicking Edit brings up selection form:

However, after selecting a change, the record is not related. It only works for Admin users.
I tracked it down to a permission issue on the Related Change Request field (custom field). After creating an ACL to allow users with itil permission to write to that field, creating a relationship worked. What I am confused about is that there are no ACLS preventing itil users from writing to that field. If I put the field on the form, itil users can unlock the field and related changes directly from the field:

Wondering if "writing" rules are different when it comes to a List field type?

Thank you.

[Ankur Bawiskar]

@fcaruso123 

which business rule you have written?

If my response helped please mark it correct and close the thread so that it benefits future readers.

[fcaruso123]

Its a rule that runs on insert/update of current change that puts the sys_ids of each related change into the custom List field. This piece is working correctly.