iphone 13 mini wireless charging stopping after 100%

only after 3 weeks of using iPhone 13 mini a problem occurred:

in the night after 1 or 2 hours after having reached 100% the battery goes down. charging is not reactivated. apple recommended to turn off background app refresh, but that does not help. cable charging works. my iPhone XS however charges to 100% and stays at 100% with same charger. charger is 20 Watt model. maybe it is an iOS bug?! hardware was tested and is okay.