Never worked for me, F-Droid provides the right APK of course. But the ROM blocks you.
No, it’s not related to the type of the app (system vs user).
Any app has this “issue”, you can’t install OVER an “older” version.
It’s a security feature, think like this, latest version has an RCE bug fixed, an enemy gets your device for a bit and to be stealthy only downgrades the app to the vulnerable version that they’ll exploit later remotely.
edit: I found a way around this on a a newer phone I have, you just need to uninstall the app and then you can sucesfully install the older version of the app.
Well…if you uninstall…the next action is INSTALL and not DOWNGRADE :))