... after the second exchange APPro worked well, there is now on one side again a unpleasant dull noise when walking/running: the "pass" test has failed since then. It's like I assumed some time ago:
Apple has apparently found a way to counteract the impact sound on the inward-facing microphone. If that fails, the footfall sounds (as in usual inEars) again audible, as in my exchanged. The seal test ("passtest" about settings, etc.) is then also negative, so an indication of the defect.
It is clearly a design flaw that affects the (error-prone) inward microphones. Please note - you hear it on the always negative "Pass" test (under Settings Bluetooth APPro ...) and the "rumbling" noise (so-called footfall - bone conduction) by the "inner ANC" yes (if they work) should be balanced.