Example of bad and foulty terminal joint
I have same problem with this green terminal.
It’s most likely not foulty, but under-tight, or the wire was tinned. You must not tin or solder the wire and you have to do them up tight.
Though I do believe that running so close to the current rating of these connectors (200x200 PCB bed is ~11A). Is fundamentally a bad idea.
I suggested Barrier terminal blocks to the RAMPS devs a while ago and they seemed open to the idea.