Okay, so I had similar issue to this before on Arch using AUR repos, and also on AVLinux which is Debian based.
Been using LMMS from KXstudio repos with Carla-Git on Ubuntu 18.04 for 5 or 6 months now and has been working flawlessly until a couple days ago...
I noticed my software updater GUI was complaining about upgrade conflicts for Carla, I put it off because I knew something bad was going to happen. Then I saw there was an update available for LMMS, and sine I have the KXstudio version I figured it would probably be safe to proceed.
After updating I opened LMMS to find Carla Patchbay and Rack plugins missing, as well as VeSTige. So I began uninstalling re-installing. In the end I disabled KX-repos and installed LMMS from Ubuntu repos, I also ended up purging Carla and re-installing it as well.
I now have VeSTige back, but no Carla plug-ins in LMMS... This is severely limiting as there are things I need from those plugs, and sure I can route Carla externally from Jack, but it has limitations.
Does anyone have a solid plan to troubleshoot this issue?