@slava77 
I think the regression has origin here.
In the old code a "muon TTRH is constructed from a SiStripRecHit2D.
The effect was that it was "frozen": ```canImproveWithTrack()``` is for ```MuonTransientTrackingRecHit``` false and its position/error was not "improved" with the trajectory directory during fitting.
Even if not explicitly commented I think this was intentional as otherwise this code would have been not effective in the very same way it is not effective now.
Easy to revert: enough to go back to the old code at line 501-509