uxTaskGetStackHighWaterMark shows how much stack was never used not the value of used stack. It should have to report as close as possible to 0 (with some margin). I see, my mistake then, many thanks.

I'm using mutual TLS with BearSSL or MbedTLS, both of which seem to use a lot of stack. Presently I've just been going into main.cpp and updating the stack size in there, I guess I can continue with ...