Tl;dr: Stereo FabFilter Plugins in C11 on mono group tracks increase loudness a lot, even when bypassed, default settings or mix to 0%. Why? How to circumvent this?
So I was messing around with a M/S template of mine and adding FabFilter plugins on the mid and side channels. I found that when I used the stereo plugins that do not have dedicated mono versions, e.g., Q3 and C2, on mono channels the loudness level of the track increased quite dramatically. I cannot say for sure how much, but it seems to be +6dB. The weird thing is, even if the plugin is bypassed and/or mix set to 0% and/or default settings apply, this still holds. Only if the plugin is removed, everything is okay (readding increases loudness again).
Mono versions of other FabFilter plugins work fine, stereo plugins on stereo channels do too, and internal plugins work well in both scenarios.
Unless I've not done anything wrong to my routing (which I double checked, and also used a MWE, without non-necessary tracks, split signal to hard panned L and R groups): Are there any ideas what the problem could be or how to fix it?
Appreciate your time and thanks for the help :)