we have 2 boards with the same problem.
there is a sketch on the board which works. after switching off and (later) switching on again, on the PC (Windows 10) i can open the port but nothing comes through.
closing the port and opening the serial monitor in the IDE shows the expected output.
after closing the serial monitor again and opening the port on the PC it works like it should.
this sketch works on a regular UNO Rev 3 flawless.
it's a serious problem for us because like it is the boards are unusable for our installation.
hopefully there's some solution or workaround.
rolf