This bug persists since I started using Notion in version 5. I write and arrange for choirs only, so this is a very annoying bug and I cannot believe it's gone unnoticed for years.
Using underscores when writing lyrics works fine in general, as long as I don't use underscores over a system break. Try and use it on a tied note that goes over a system break. The underscore will always stop at the last note before the system break. No matter how many _ _ _ _ you type into the lyrics.
So far I switched to MuseScore, where this works perfectly. I'd love to be able to continue using Notion though.
Please please please fix this annoying bug.