The battery chemistry and how it is being regulated is important. UPSs based on lithium chemistry (various different kind of lithium chemistries exist) have begun to emerge. Assuming lead acid batteries, there are important parameters for lead acid batteries: for charging and standby. Charging does not happen all that much (I hope): standby most of the time.
With lead acid batteries, fully charged during standby, it is very important that the correct floating voltage is applied. That floating voltage mostly depends on the type of lead acid battery, that may differ per brand specification for that specific type of lead acid battery. The optimal floating voltage is also temperature dependent. If the floating voltage during standby is too high, (more) water will inevitably evaporate. Various battery designs exist to minimize this evaporation (e.g. VRLA), so this is not a quick process. However, water that has evaporated is gone—no filler caps to add demineralised water for most UPS batteries
I have no specific experience with UPS batteries, but it would be interesting to know the floating voltage during standby that is being applied to lead acid batteries in UPSs. Can the voltage being applied to the battery (while charging and during standby) be read and values be logged? It would be ideal if the floating voltage can be set/regulated via a software interface.
UPS firmware regulates charging and standby (floating voltage), so it could very well be a "firmware thing".
If you have no specific data about the floating voltage applied during standby by your specific UPS brand/type hardware, then it might be beneficial to select a replacement battery that has at least the same floating voltage specification (perhaps even a little bit higher) than the original batteries, if possible.
Because of the nature of an UPS, there's always a certain trade-off between the biggest standby capacity (how long will my system be able to run without main power from the grid) versus the longest battery life. A slightly lower or better regulated floating voltage (by your UPS firmware) during standby is more beneficial to the life expectancy of your batteries but it may shorten the minutes of active duty during a power outage.
Some lead acid battery documentation:
Charging Lead Acid; more on
large UPS batteries.