Could not select some LAG-interfaces as parent after upgrade

[nomad-cyanide]

After upgrade to 3.1.9 from 2.11.4 I have come across a weird little error.

We have a number of LAG-interfaces and a lot of virtual interfaces with one of the LAG-interfaces as parent. After the upgrade, some of the LAG's could no longer be selected as parent for new virtual interfaces. It was on the LAG-interfaces that has the most child-interfaces (90+) that this happend for. The LAGs with only a few children did not show this behavior.

Has anyone else seen this error?

It could be fixed by simply editing the LAG that cannot be selected as parent and saving it, without making any changes.