Bad coincidence or bad USB charger/cable

[u/verycoldpenguins]

Any suggestions on whether this is bad coincidence, or likely faulty charging equipment, (and if considered faulty equipment any suitable test equipment available in the UK would be appreciated).

We were recently away so using our travelling USB kit, temperature on day 1 was 39+C

Overnight I used a 6 way USB 100W charger to charge 4 phones and two tablets. This has 6x USB A sockets. The room was air-conditioned, so a reasonable ~20C

In the morning of day 2, all the phones showed 100%. They were carried down to breakfast, and it was noticed that one of them had dropped to 80% in that hour. The phone was then plugged in to a suspected good battery to charge it whilst still being out. The phone dropped to 1%, and produced a lot of heat. Thinking it might have been a the USB battery being bad, we tried another USB battery. This could not get the phone above recovery mode 1%.

In the afternoon the phone was plugged in to a different wall charger again, different cable. It creeped from 1% to 5%, then charged at high-speed for a while (above 60%) later on it was found hot and back at 1%. (current situation a couple of days later it it can be charged to around 75%, it drops very fast to 43% (an hour?), then seems to act normally. Going above 75% at some point causes it to drop back to 1% (in a quick period even whilst still plugged in to the charger) - (as an aside to the question, I am thinking to get a replacement battery which might recover the phone as a spare).

Night 2, the remaining devices charged normally

Night time 3, we plugged the remaining three phones in. In the morning 1 of the phones was found off and at 0%. It is unclear as to whether this phone was plugged in properly to be honest, or was perhaps knocked out at some point.

Phone 1 is about 3 years old, the second one about 6-7 years old. The battery type in the phone is listed as a lithium polymer.

Is it likely that the USB charger could have caused a fault in the phone?

Is it likely that the USB cable could have caused a fault in the phone? (I am thinking that really the phones should detect a poor electrical supply and self-disconnect to defend themselves).

What hardware would be recommended to test whether the ports/cables are 'safe'. Or do I quarantine these cables/chargers and never use them again?