There are some reports about a fire/smoke/fumes issue with batteries failing on the higher capacity models, this exact model included. See https://www.tomshardware.com/news/cyberpower-upses-reportedly-pose-fire-hazard and https://www.tomshardware.com/news/cyberpower-ups-smoke-reports-continue. The consensus seems to be that it’s only older (2017-ish) units that are an earlier hardware revision, and with their original batteries. The batteries in this Woot! refurb should be new/refurb (per the description, “The batteries have been replaced by the manufacturer as part of the re-conditioning process.”), but some comments in the articles make it sound like it’s the unit that’s at fault - it doesn’t read the battery voltage correctly, so it tries to overcharge them.
Per the original YouTube video: The problem was/is the glue used to reinforce components attached to the circuit board(s) – the thin metal leads could break from vibration during shipping, so they throw varying amounts of glue on the boards. Over time that glue starts to decompose, becoming both corrosive and electrically conductive. I can verify the problem having partially disassembled an almost 4 year old Cyberpower UPS, finding little of the yellow glue, but what was there had started eating a hole in the circuit board. Personally I think it’s extremely unlikely that CyberPower changed their manufacturing techniques, &/or started using a more expensive, higher quality glue in 2018 or later – if a newer 2021 or 2022 model hasn’t had the same problem it’s because it’s not old enough yet. In the video that started it all the OP picked away the old glue, and repaired the units that didn’t work properly – he apparently had several – so that’s one solution, while another would be to just treat these as having a 3-4 year lifetime. FWIW, I started looking for a more expensive UPS from APC, until I read reviews on Amazon of them catching fire.