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These are clearly not the same device, see the major/minor? Another device is detected as a prolific serial converter and is taking the symlink. You need to be more restrictive on the device selection in the udev rule.
I only have one adapter plugged in, it's a symlink through udev rules.
Something has changed, I need to figure it out. yes I agree the major an minor are different.
Change logging level of the udev daemon with "udevadm control --log-priority=info". Plug out, run "dmesg -Hw", plug in. You should see what is happening perhaps.
Oh sorry now I see it! You need to filter the "tty" subsystem in your udev rule, not the "usb" subsystem. What you are interested in is the device that offer serial transfer features...
Just FYI for the people googling this in the future: I had this exact same problem with a CP201x usb-serial adapter. The solution is to change all the "ATTR" references to "ATTRS" in the udev rules. That's it. That's the magic incantation. I hoped this helped someone.
In this case changing ATTR{manufacturer}=="Prolific Technology Inc." to ATTRS{manufacturer}=="Prolific Technology Inc." most likely would have fixed OP's issue.