I had a similar problem with the udev rules too.
It seems that the BP5 has 2 serial ports with the same serial code for the same idVendor and idProduct. When you connect to USB the BP5 sometimes picks up the right serial port and sometimes picks up the wrong one.
Not having found a solution (I’m not an udev expert) I configured everything to always work with /dev/ttyACM0
This is how it always works.
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