Curious quirk in RS232 comms between two COM ports.

I have a test setup in which I have two COM ports, COM1 and COM2.

Between the two com ports I have a fully wired null modem cable.

I was testing a python script meant to parse HPGL commands received on COM2, while I send a file to it via COM1 (Shell: copy x.plt COM1). This is all on the same computer but I don't see where that should be a problem.

The problem is that any first try at communication fails with garbled characters. Only when I change direction, that is, python script listening on COM1, and sending the file via COM2 does the communication suddenly succeed, up to tested speeds of 57600.
I thought I'd solved the problem but alas, as soon as I changed settings or replugged the USB adapters (PL2303), the received data was garbled again. I have to change direction again(2->1 and vice versa) for it to work. This pattern is rock solid and I cannot seem to find where the cause lies. Two other, brand new adapters gave the same result.

I have not tested between two different computers, but I am half expecting that to work for some reason and would be a lot of work for me to set up. Could it be that both adapters are identical? I have unique ID set to on in the settings.

If anyone reading this has any experience with this , please drop a reply. Thanks in advance.

Of course, as I use the name COM ports, This is on a windows 11 machine.