Hi, I'm running MusE 4.1 appimage.
When I don't have the package lv2-dev installed, then the right-click synth menu contains no lv2 synths under the synth section. Those are found in the "effects" section.
When I do have lv2-dev installed, the lv2 synths are correctly found in the synths section, and not in the "effects" section anymore.
Okay, then just install lv2-dev, right? Well, I had that installed lately, but today I found out that having that package installed broke nearly all my older MusE projects (all created before I had lv2-dev installed), in the sense that loading a project led to various lv2 plugins being reported as "cannot be found", even though they haven't moved, and I can just add these very same "missing" plugins on new tracks! (Affected lv2 plugins were pianoteq 7+8, odin2, calf monosynth, all avl drumkits, maybe more)
That would not be so big of a problem if one could just switch synth plugins on an existing track, while keeping the effects rack and all automation. But alas, that does not seem possible!
Because I didn't yet know that lv2-dev was the culprit, I restored an old (december 2022) snapshot with Timeshift, and brought my system back up to date step by step. Thankfully I found out the cause, and my projects are still accessible without the offending package!
Edit: And my new projects, created with lv2-dev present, don't seem to be affected by its removal. Phew..
Edit 2: Except for Fluida.lv2. The prebuilt binary doesn't work on my system, so I built that myself. One of its build dependencies was lv2-dev, though, and removing it crippled the plugin too.