Hello Peter,
please see this topic and try to verify it with add followed code
printf(" MbedOS v %d.%d.%d\r\n", MBED_MAJOR_VERSION, MBED_MINOR_VERSION, MBED_PATCH_VERSION);
You probably will see MbedOS 5 .15
I solved that with downgrade to the MbedOS 6.2.1, until it will be solved.
BR, Jan