Why does my UPS shut down unexpectedly during brownouts?

My neighborhood has frequent brownouts that can last several hours, so I bought a CyberPower UPS rated at about 1500VA/900W last November. It worked normally at first, but it now shuts down unexpectedly even when the load is only around 140W with two monitors connected.

During one brownout, the UPS began beeping as if the battery were nearly empty and then switched off. Power returned about 10 minutes later, and the UPS briefly appeared to be charging before reporting that the battery was already full. On another occasion, there was no apparent brownout, but the UPS started beeping and shut down anyway. The monitoring software shows the battery charging, stopping, and then immediately reporting 100% capacity. Its logs also show several periods where the UPS lost communication with the computer.

I tested it by unplugging the UPS while the PC was running. The software reported the battery capacity dropping from 100% to 12% and then 0%, although the UPS did not shut down immediately and began beeping continuously. Does this indicate a failed battery or another UPS fault, and should I request a warranty replacement?