Hey there Geek_pi,
Being able to save energy by putting the display to sleep when it isn't needed is a great way to avoid additional utility costs and device wear. Let's look into ways that we can get the display to behave more how you expect.
From what you mentioned, this began after updating to macOS Catalina. Since then, have you made any other changes or additions as far as software goes? This might be important as if an app is using resources for the display in some form or fashion, that could be keeping it on.
If nothing else has changed, you might want to try restoring the default settings to the Energy Saver page of System Preferences and then setting them up once again to see if this will achieve a positive result. To do that, go into Energy Saver and tap Restore Defaults. From there, just configure the items how it suits you best and do a little testing.
Let me know what you find out.
Cheers!