I've just run the Python script that calls arduino-cli but only for the own-build ESP32 NINA-W10 board and that has worked, so the issue seems to be simultaneity, we are not able to run two copies of arduino-cli at the same time successfully. Unfortunately all our test scripts start at the same time, running in parallel on different boards, employing different tools (ST, SES, west, nRF, JLink etc. as well as five instances of ESP-IDF), to minimise execution time.
Any idea what the cross-connection might be in this case?