Pianoteq is a proprietary commercial product and supports LV2 on Linux. Pianoteq 8, which was released recently, does not work fine on Zrythm, while Pianoteq 7 works.

Precisely speaking, Pianoteq 8 still plays notes on the preview keyboard (which is placed on the left side of the pianoroll). It also plays when we expand or move a note on the pianoroll. But on playback it does not work, as if no note event happened.

I would describe why this could happen a bit in details.

They have been supporting LV2 for a while, and if I'm not mistaken, it is based on JUCE. Pianoteq 7 is probably based on DISTRHO/JUCE (haven't confirmed, might be different exporters) and the plugin parameters are mapped to lv2:ControlPort. But Pianoteq 8 seems based on JUCE 7.0 which supports LV2 natively.

There are many differences in the actual exported implementation between those two. For example, their native LV2 build treats those plugin parameters as its own parameter patches and the parameter changes are sent via the atom input port. It is just one difference and there would be a lot more, but you'd find that it is quite different.

It is however not as simple as JUCE7 LV2 plugins do not work at all. I tried Dexed, adding LV2 exporter and LV2URI to build, and tweaked the resulting dsp.ttl a bit, then it worked perfectly on Zrythm.

At this state I have no further idea why Pianoteq 8 does not work. As far as I saw on tootverse it does not seem to load on Ardour, but Reaper and Carla seem to load it (dunno if playback works). The JUCE7 approach seems valid and LV2 community seems to welcome their approach so I would hope the JUCE7 plugin could be loaded, or at least we could identify what's going wrong and how to workaround it (while I'm not particularly a Pianoteq user though).