Mines been doing very similar; leaving it on my desk with 95% charge and then getting to it in the morning either completely off or with around 30% charge, and then it'd knock off, completely dead!
Completely reset the phone, ran it with native OEM apps only, did exactly the same, ran it in safe mode as per online Google instructions, exactly the same
Spoke to Google last week and they're sending me a like for like replacement which should arrive tomorrow, according to tracking...
First thing I'll be doing is checking battery health with Acubattery and then running the same tests...
Great phone, terrible battery issues unfortunately