Plenty of technical reasons can explain that. For example, a UPS must specify (with a number) its switchover time. That computer must define how long it can still operate with no incoming AC power. Yes, all electronics will operated uninterrupted without power for a given time. A minimum is about 17 milliseconds. Better electronics routinely operate without power even for 100 milliseconds.
A power outage benchmark is a VCR or microwave oven clock. These tend to detect a power loss (end up blinking) much more easily than anything else. If that clock did not detect a blackout (or longer brownout), then power loss probably did not happen.
View the UPS. These things are often designed as cheaply as possible. Battery life expectancy is only three years. Noise and other anomalies can trigger a UPS into thinking a blackout has occurred. Some even get confused when power cycles on and off, or when voltage keeps rising and falling between a fixed threshold.
Possible that a UPS kept assuming power was there when in fact it was not. Or worse, got confused and switched off both AC and battery power due to that confusing power cycling.
That same 'confused' power cycling is why better computer systems have a protective lockout. Rather than trying to decide what is and is not good power, a computer simply locks out power. That lockout gets reset by disconnecting its power cord for less than 10 seconds.
(Some use speculation to assume this is letting a computer discharge. It does not. Power cord is disconnected to reset a hardware safety function.)
This strange UPS behavior is why a manufacturer, model number, and relevant specification numbers are necessary to obtain a better informed reply.
Same applies to a battery. How does a UPS know that battery is fine? A defective battery can still measure good. The fact that the battery did provide power for sufficient time implies a good battery. However, was that battery fully charged? A UPS is made as cheaply as possible. A power source that does recharge is made as tiny as possible. So a UPS battery often needs at least 12 hours and sometimes 24 hours to be recharged. Was that battery sufficiently charged to support a small load, but not a full load? Again, an informed answer means perspective - that means numbers.