I get wonky readings on all cheap meters, but especially with the cheap leads they come with. Cheap meter/lead combos tend to read fairly well above 24 volts, but I've never had luck with them in our uses. I've returned some 6 cheap meters (Klein from HD, Southwire from Lowe's, and something from AutoZone) because there was just too much resistance inside the meter.
My old B&K (20-30 years old) agreed with my d4 about battery voltage within .02 volts, so I think it's golden, and I suspect any actually Nitecore charger with a voltage display would be very similar.
OP said batteries were charging to 80-85%. Does this mean a particular voltage?