I have earlier written about bad cheap Chinese wire connect that did not live up to specifications and was potentially dangerous for mains power at
Here is teardown of that
Let’s open open it
Here was all. The problem in construction that there is quite weak spring that pushes against wire. On the other side is just plastic that melts easily if there is heating. The contact area is small and spring is so this msterial that things start to heat up considerably at current more than few amperes!