I've run some tests and debugging sessions; I noticed a few problematic behaviours with the Calf plugins but they seem have a common origin.
I had both calf-plugins *and* calf-ladspa on my system, both containing calf.so with a subtly different implementation. The latter packages comes from lmms. I removed calf-ladspa and now it seems to be ok. I have not tried the Debian stretch package of ardour5 yet, I'm still running my newer 5.9.0 build.
