Notes Before Rest

I am working on a guitar piece that has two guitars. I finished the first guitar and now working on the second. When I add a rest or change a rest value, notes before the rest will either disappear or the note value will change. Is this normal? If this is how the software works, can you freeze what comes before the rest so it isn't changed? It's fairly easy to fix but just really annoying to have to go back and fix notes or add them back.