We discovered a strange behaviour .
First the simple setup : a master record is connected to a related record by ID and a relationship that allows creation of related records.
A layout is based on the master records' TO and has a portal showing related records. (allow creation of related Rec. is enabled).
Create a new Record. Enter data (manually) in related records' fields and commit. Fine.
Run a script, that creates a new record, then sets a field value into one related records' field, and you get the same result only if the current account has [full Access] privileges.
Expected result: any account that has full create, edit, delete privileges in both tables should tun the script, create the records and allow for commit.
Observed result: only [full Access] accounts can run the script successfully.
I do attach a demo file that demonstrates this. While the file is open (Admin - by default) you may change the login and watch the different behaviour.
(FMP 184.108.40.206 on MacOS 10.10 and MacOS 10.11 tested on several different machines)