found a workaround.
the problem is somehow related to graphic interface of Altiverb. snapshot changes stop to read only if I use Altiverb UI to switch scene with mouse click.
Switching to generic editor Gui and moving parameter there unblocks it.
I made quick control to parameter "autopres’ and I can switch scenes with Nuage (it’s incremental but works with fill loop) even with Altiverb GUI.