FortiExtender stops working on FortiOS 7.4.6

[Magnitude_8]

I've discovered what looks like a bug in FortiOS 7.4.6 and am posting this in case anyone else runs into the same issue. It's not currently listed as a known issue in the release notes.

After upgrading to FortiOS 7.4.6 on our FortiGate-80F firewalls, the connected FortiExtender stops working. The FortiExtender goes offline on the FortiGate. You can't re-authenticate the FortiExtender and if you delete it you can no longer add it again. This looks like a CAPWAP bug.

This is reproduceable on FortiGate-80F firewalls and downgrading to FortiOS 7.4.5 resolves the issue. FortiGate-60F doesn't seem to be affected and I haven't tested any other models.

I've logged a case with Fortinet so hopefully it appears as a known issue soon.