Problem is caused by water getting past the plastic gasket between the handle and the tailgate. One or more of the studs rusts as a result, the rusting causes the stud(s) to swell and in turn fracture the section they are moulded in to.
To help prevent this, put a coat of sealant on the gasket to prevent water ingress which can also cause the tailgate to rot just below the handle, above the number plate.
If fitting a new handle, give the three studs a good dosing of Waxoyl or grease in addition to sealing the gasket.
Finally, when fitting the handle, don't go overboard tightening the 3 nuts thinking it will squeeze the gasket tighter...you are putting more strain on the studs where they are attached to the plastic and they could come out.
I speak from experience of having to replace both a rusted tailgate and handle !!
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