OK, so I searched a bit on how crossfading between two signals is usually implemented, and there are basically two ways that are used: a linear (or constant gain) or an equal power crossfade.
It seems that Tube Compressor uses the second approach, which applies a non-linear function to the signals.
See here for a rather nice, visual explanation:
So it isn’t actually a bug, but simply a different implementation of a crossfade (although one that - from what I read - is a bit unusual when crossfading two correlated signals)