Before replacing the turn signal switch, try this:
Push and pull the hazard switch button several times. Sometimes that will get weird taillight problems solved. If you look at the electrical diagram for the turn signal switch, you will see there are a LOT of separate contacts that have to open and close to get the different functions to work correctly. It’s kinda amazing turnsignal switch problems aren’t more common if you think about it. Sometimes you get a little oxidation on the contacts and things don’t work the way they should.