+1 I can also reproduce this!
Edit: Looking a little closer at the mimeapps.list because of URL Scheme doesn't work on Linux, it turns out that opening Obsidian makes it the default application for html files, instead of the browser. I don’t know if this is related, but this is definitely a bug. I don’t think Obsidian is intended to open other local html files, but even if it is it should be added to the [Added Associations] section instead of making it the default.