suil 0.10.16-1 breaks LV2 plugin GUIs in Ardour, you can see the original Arch bug report here:
I found suil 0.10.16-2 has a different issue where plugins can only open once in Ardour, best solution is to downgrade back to suil 0.10.12-1.
sudo pacman -U https://archive.archlinux.org/packages/s/suil/suil-0.10.12-2-x86_64.pkg.tar.zst
And…Welcome to the Manjaro forum. 
Never fun when your first post is trying to solve an issue. Hope this was helpful.
Always beneficial to check out the latest Stable announcement thread, there is a wiki post that follows where everyone can contribute solutions to issues recently updated packages cause.