Hmm... and if you substitute a MOTU plug for FabFilter, I'm guessing it goes away? Might be worthwhile to report this to MOTU to see if they can track this down. I've had a couple of little issues with third party plugs in DP that don't seem to happen in other DAWs. One involved a VI (the plug was not native) that could not see incoming MIDI data played on my controller keyboard or from a MIDI track targeting that VI.
Come to think of it, it was probably related to DP's "mixed environment" that can run native and non-native plugs in the same project when DP is launched AS-native. If I launched DP in Rosetta2 there was no problem. And of course, Pro Tools had no issue but then Pro Tools is not AS-native yet. I hardly ever use Pro Tools, but even still... Avid isn't it about time you got your you-know-what-together and released a native Pro Tools. Especially since you're moving your users to subscription and charging a premium?