Hey, I’ve tried using URIs on Pop!_OS and it didn’t work on chromium browser but works on firefox(as you said). I think this is got to do with chromium browser not being the default browser. This is similar to how your Syncthing web GUI won’t open on chromium if it isn’t the default browser.
I just added a few steps here to set up the Obsidian URI… maybe double check what xdg-mime and xdg-settings output. It might also depend on how you’re setting your default browser too (I use Firefox as my default, so I can’t help with the Chromium part unfortunately).