Question

Convert to H1 ~ H3 with ctrl+shift+1~3 works well but converting to normal text with ctrl+shift+0 does not work.

Answer 1

The only thing I can think of why this is not working in Notion and assuming you are on Windows, it is you keyboard key for the cero.

Try the other 0, you have top numbers and keypad numbers at the right, try with the one you are not using to see if that one works.

Also confirm if this happens using Notion in a Browser or in the Notion App, because it may be the Browser config as well if you are using it there.