Bonjour, Doupirate!
If you found damage to the ball bearing, and/or damage to the inner or outer races, how would you differentiate the damage that happened during trailering on the road from damage that happened while the trailer was static in storage?
Actually, grease does not melt like oil over time, rather the oil separates out from the fatty acid, soapy carrier component that is its thickener. The grease for trailer bearings is necessarily thin (as greases go) to minimize internal flow friction and prevent heat buildup in use, which is a big enemy to bearing life, plus heat accelerates the thinning and thus shedding of the lubricant.
Even when sitting stationary, over time the oil separates out from the fat carrier and leaves the carrier behind on the bearing surface in its place. (Witness the oil that leaks from a grease gun that has been left sitting too long).
Now the thick carrier is not at all a good lubricant, its job is to hold the oil (which is a good lubricant) contained in it in its place at the bearing contact surface, and to help keep water out and trap dirt, among other things.
Even with bearing buddies (which are good), giving the wheel a spin by hand serves to redistribute the separating components during storage, and gets the oil component back onto the bearing contact surfaces where it is needed the most.
I always prefer to start with a short answer, as I did earlier. But if that’s not sufficiently clear enough I’m glad you asked so I can provide you with a more detailed explanation, so I hope this is helpful.
Regards - Brian.

