I am not sure if Obsidian follows XDG specs, but could you try xdg-open https://example.org/ in your terminal? Anyways XDG is the only spec I know that should be respected by distros and linux apps in general. So, if xdg-open will open your default browser, this is indeed a bug.
I am not familiar with snap, does it containerize apps somehow? If yes, make sure that it has access to your system configuration about default browser.
And lastly, provide, how do you set your default browser? Do other apps respect your setting? Telegram for example, please provide examples of apps which respect your setting.
For me it opened chromium even though I have firefox set as default browser. Running the following commands in the terminal (and then restarting Obsidian) made it work for me: