Keep in mind that Reinhardt’s voice actor can’t speak German, so he also pronounces it incorrectly. These videos show how a German person would say it:
True, it’s not how German’s would say it but there are other ways of pronouncing it. Kind of like how Ana can be said in two different ways.
This particular character uses the French/Dutch way, and maybe there’s a lore reason for why Reinhardt would choose that version. To me, it’s a prettier sounding way of saying it too!
Technically Orisa can be said in two different ways too. It really depends how you want to pronounce the ’ i ’ in her name and I suspect a lot of Americans lean towards it being pronounced the “wrong” way.