At least that's what the Wiki says, it doesn't seem to be a Bug, just "normal in game tree behaviour", some trees just don't change back...
On the day of the Green Rain weather event, approximately 75% of trees have a chance to turn into one of the varieties of Green Rain Trees. These trees revert back to their original form on the next day.
All three types of Green Rain Trees can spawn naturally during the Green Rain event, can grow from Mossy Seeds, or can have a fully-grown Common Tree grow into one.