When adding lyrics, two tied notes are treated like two separate notes.
1) Add lyrics. When you type the lyrics, a space character takes you to the next note. However, for tied notes, it treats the second note as a separate note - even though it's tied and wouldn't be played separately - and puts the cursor there. It's very easy typing lyrics to miss that and find you're on the wrong note. It may be there's a switch to change the behaviour!
What would be brilliant would be having a syllable dictionary so you could select the first note of a phrase and paste the lyrics with Musescore putting a syllable per note, but maybe that's asking a lot.