FabFilter

There is a strange behaviour on protools: when a source bus for sidechaining is deactivated (and so that any track have it activated in the session), then the plug-in's internal AAX side-chain is not working anymore and the SC is feeded by the normal input. I would expect to just have no sound on the SC bus input...it is particularly weird on fabfilter C-1 as it can be still set on "EXT" visually.
duc.avid.com/showpost.php?p=2710629&postcount=20

a better link to see the whole discussion about that duc.avid.com/showthread.php?t=391060

If the plug-in is set to external side-chain but there is no source set for external side-chain, the plug-in will use the internal side-chain instead.