Juste tested disabling Intel Dynamic Tuning Service
It works...but it completely disables power management/throttling and the CPU runs constantly at full turbo speed (thereby dramatically reducing battery autonomy).
So I reckon not a good workaround.
But this conforts me in the idea that this problem is a driver/firmware/software policy issue, NOT a hardware malfunction.
It could be an immature implementation of the 'Intel Dynamic Tuning 2.0 with Machine Learning' mentioned in Barb's link
So Intel would be the culprit ???? Perhaps an update of the Intel Dynamic Tuning Service is needed ?
IMHO the user should always be able to override AI tuning decisions, e.g. by sliding the Power cursor to the right; forcibly crippling a computer at 35% battery power is ridiculous.
And testing again today, I see that indeed the symptom is independent of any connected hub/keyboard etc. Not sure why during my first quick t'shooting tests I came to the conclusion that it was only when an external USB hub/keyboard was connected: it is
NOT the case.