This happened to my Noibat. I was battling the woman you first see on Route 6 ( sword/shield) and then her Clefable used Forest Curse, making my Noibat gain another typing. So, what types it now???
In your Noibat's case, it is Flying/Dragon/Grass, due to Forest's Curse. This means Ice moves are even more powerful against it, and Grass moves are even weaker (among other new type interactions).
Hope I helped!
Noibat would be a Flying / Dragon / Grass type. It would be Flying / Dragon / Ghost if Trick-or-Treat gets used against it.