BINGO!!! @Fredo 
@Dirx has given the hint to the crucial variable! I can confirm that the error described in my messages and tests is indeed and reliably dependent on the position of the plug-in in respect to the fader. Multi-track routing for post-fader plug-ins is recalled correctly only in slots 11 and 12. In all other slots, the routing is reset to default in a post-fader position, regardless of where the fader is placed.
… to make things even more complicated, even pre-fade multi-track routing is NOT recalled properly in slot 11 as soon as the fader is moved to the position between slots 11 and 12.
Here is a simple test project that show-cases the issue:
NU11Routing Test4.npr (452.8 KB)
@Steinberg: Please fix.