Qtractor with SCVA work not well.

may you explain what you mean with intermittently ?

and how and where you're inserting the plugin? you should know that for multi-timbral instrument plugins like gmsynth.lv2 you'll have to insert it on an MIDI output bus, don't you?

seeya

Qtractor is a DAW and a plugin host.
Carla is a plugin host and a plugin, a swiss army knife with advanced routing and soundfont playing capabilties.

Different tools, different features although they have some features in common.

I think I found the culprit: it is the source .mid files you're using that are troublesome or/and not quite well-formed if not simply disfunctional; at least for the Qtractor's already 2 decade old MIDI file parser, sequencer and renderer...

the root cause and most probably the problem seems to be some of your .mid files are showing contiguous sequences of note-on, note-off and then note-on events at the exact same time (ie. zero delta-time); although this might work fine for external/outboard MIDI instruments, it doesn't at all for internal ones, that being plugins, no matter their type (LV2, VST or else).

for example: "02 - Freebee.mid", track/channels 3 and 4 (might be others): most note events are simply bad because of the above reason--if you just eliminate or strip those note events from the file--note events with zero duration--they will all start playing fine (in plugins).

so it seems it's a bug in the original MIDI files and not Qtractor per se

although Qtractor could cope with that alright, it must be here said that those zero-duration-note-events are actually and utterly non-sense and should be sanitized at the source.

cheers, hth.