There must be something wrong with contact between traces/solder/connector/wire on this unit. May be many things, but high resistance is basically just a bad contact/too small area of contact.
For instance, one time an SKR board came to me with bed connector not soldered(!) to the board. I didn’t notice for quite some time and have been using it, because the connector was seated strong enough. I noticed only because of the gradual heat damage to the board, and bed heating times getting longer and longer to the point of getting thermal runaway errors. Only then I inspected it in detail.
The other time on another SKR board, the connector was soldered, but slipped out of the solder which was the first time in my life I’ve seen anything like this. The solder cone was correctly formed, but I could slip the connector in and out of it. Guess something was wrong with surfaces of the connectors pins, maybe wrong metal on the outer layer so it didn’t really get wetted by solder, guess I’ll never know. Thankfully I’ve noticed this right before assembly.
Do not trust cheap DIY electronics that are suppossed to run long term unsupervised unconditionally, is what I’m trying to say.
Exactly, I agree with all of the above.
Low DC voltage won’t shock you, but it sure will try to burn your printer down. The lower the voltages, the higher the heat losses in critical points (with the equivalent power ofc).