Gave it a quick one from the terminal in Linux Mint 20.1/Cinnamon, Obsidian/AppImage 0.11.13, Installer 0.10.11:
xdg-open "obsidian://new?path=/home/matthias/Dokumente/Schreiben/xyzzy.md"
works,
xdg-open "obsidian://new?path=/home/matthias/Dokumente/Schreiben/+Inbox/xyzzy1.md"
doesn’t work.
So I see the same behaviour as you do.