That is a big enough difference in voltage that I'd invest in a second charger and battery checker from different brands. (Not that that is affordable by any means. Lol) I have the same battery checker and it seems accurate. I run this charger which also seems accurate: HTRC Lipo Charger 1-6S Touch Screen Dual Discharger AC200W DC400W 12A T400 Pro High Precision Fast Balance Battery Charger for RC Li-ion Life NiCd NiMH LiHV PB Smart Battery https://www.amazon.com/dp/B089SZBVS9/ref=cm_sw_r_apan_i_QB2QAABVPJ16KR7E64RV

These are also good options:

Venom Pro Quad LiPo Battery Fast Charger | 4 Ports at 100W Each | AC DC 7A Fast NiMH LiHV LiPo Balance Charger Discharger https://www.amazon.com/dp/B01HZ1M2FM/ref=cm_sw_r_apan_i_22V6YR57X16A2C94N8YD

https://www.amazon.com/dp/B087CJZTXJ/ref=cm_sw_r_apan_i_B15MXMHQHQDVPXD1XMKY?_encoding=UTF8&psc=1

I'd say your charger is the issue, and you are not getting fully charged batteries. That may lead to running your batteries too low. I'd start with a new charger, even if it costs a bit. For the price difference, you might as well get a dual charger these days.