I thought I had a finicky MKR1000 because I was always getting a port busy error, so I bought a new MKR 1010 and its doing the same thing. I've tried 5 different USB cables and two different USB ports.
I can upload a sketch if I double-click the rest button, but then as soon as the sketch is done it reverts back to the original COM port and opening the serial monitor always gives me Port Busy.
I've tried rebooting the computer and the most I can get is for it to work one time.
BTW, This doesn't occur with my NodeMCU or Wemos D1 projects, just the MKR's.