Hello!
We have 4 Eaton UPS and none of them report incorrect voltages for Input, Output, Bypass and Battery.
Eaton is compatible with RFC 1628 for the models we have (9130 UPS and PW9130 3000VA-R) and LibreNMS collects voltage data from this MIB OIDs.
What model do you have? What values do you see via SNMP?