@martinm
What are you using to determine that it is coming out of your plugin chain close to 0dBFS, and what are you using to determine that it is less after the fader? And how much less?
So far I haven’t found the -18dB value @RS mentioned anywhere in the manuals. I do see that 0db on the meters is +4dbu analog, and the maximum output is +22dbu. Those in combination indicate that 0db on the meters should correspond to -18dbfs. However, but that is the meters not the fader. (And then there are the technicality questions about whether the meter ballistics are sample peak, true peak, RMS, or some other thing).
I also see in the manual where there are “analog” and “digital” settings for the inserts, but my reading of the manual seems to indicate that those affect the insert send level and not the return – so in your case it doesn’t seem like it would be what you are looking for.
Intuitively, the fader at 0db should mean just that, 0db gain. It appears from the block diagram that the matrix inserts are followed by several other blocks in the processing chain, so make sure there is not a gain change in any of those. The block diagram also shows a global meter setting. It might be helpful to try changing that setting to see what your master meter looks like post-insert, post-eq, post-comp, post-fader, etc.
Also be alert to the possibility of errors or inaccuracies in the block diagram or manuals. I have seen those before in other equipment at least 🙂