If a populated mandatory Glide List field is made readonly by an onLoad UI Policy, the red asterisk still shows against it.
If it has a value the asterisk should be turning grey.
It being red does not affect the record being saved though.
Release or Environment
This is reproducible in builds up to and including New York.
Problem PRB1181534 addresses this behaviour.
PRB1181534 is fixed in the Orlando release.