I guessed the behaviour is by design because it’s been there since when the ability to open new windows arrived. This could absolutely be a bug. I checked the forum for bug reports and other kinds of posts regarding this but found nothing; I (perhaps foolishly) assumed that others would have reported about this being a bug before me.
That’s true, it’s surprising that a bug that interferes with such a core feature wouldn’t be reported yet. Maybe something in your setup is causing it — do you see the same problem in the sandbox vault (found in the help menu)?
Aha! So the problem is probably a plugin — maybe one that hasn’t yet been updated to support extra windows.
Plugins don’t update automatically, so if you haven’t been updating them, go to Settings > Community Plugins, click “Check for updates”, apply them, and restart Obsidian.
If that doesn’t fix the problem, you’ll need to figure out which plugin is the problem. If there have any obvious candidates, try turning them off one at a time (restart Obsidian each time to make sure they fully unload). If you have a lot of plugins to check, you might want to use the Divide And Conquer plugin to help investigate.
Oh, thanks a LOT for your help! All plugins are updated, so I’ll check them all later today to try and find the problematic one. Also, thanks for suggesting the Divide And Conquer plugin. I’ll report back.