as if I turned the UPS off myself with the power button.
Contact APC and ask what the issue is.
With other APC UPS models the power would run for much longer than with these ones
If you pay peanuts - you will get monkeys.
These two UPSes, are almost bottom of the barrel, with standby topology (cheapest topology there is). Slight saving grace is that they output simulated sine wave and not square wave, but for PC's, simulated sine wave isn't good either.
Proper UPS, for PC's use, would be true/pure sine wave, with line-interactive topology (e.g like my CyberPower CP1300EPFCLCD). Perfect would be double-conversion topology (aka online).
Also, it depends what capacity UPS you have. Your UPSes are 650VA/400W units, while my CyberPower is 1300VA/780W unit. 50W load on your UPS lasts for 60mins, while same load on my UPS will last 80mins.
Of course, UPS isn't meant to be used as a generator/battery where to run your hardware off from constantly. What UPS is for, is when main power cuts (blackout), you'd have some backup power to safely shut down your PC, without data loss or damage to your hardware. If you want to keep your PC running for longer periods of time when there's no main electricity, buy a generator (gasoline or diesel powered).