I've had my Fenix 5 Plus swapped once during warranty because of this. And the replacement's contacts started to corrode again when I sold the watch. It might not happen to all models, but some are very prone to it.
They make little ~rubber caps to cover the charging port when you're not using it. That, and make sure it's dry when you charge it to rule out galvanic nonsense.
Just have a look at Google results: https://www.google.com/search?q=garmin+watch+contacts+corrod...