Thanks for that.  As you probably know,  the charge rate decreases as the battery becomes more fully charged.  What was the battery level when you first started to charge it?  

Battery level was 20%. It actually started at 8 Watts for about 30 seconds and settled in at 5 Watts. Toward the end it dropped to 3 Watts which I expected it would drop. I didn’t monitor pass that point.

Fascinating.  Bog standard USB 3.0 charging is capped at 4.5 Watts

I wonder if there is an issue with the PD negotiation between your Pxwaxpy charger and the headphones. Or perhaps the Baseus cable with its embedded power meter is causing an issue.  However, the charger started at 8 Watts, so some negotiation occurred. Perhaps this is expected behaviour but it’s hard to believe Sonos would implement PD charging only to cap it at USB 3.0 charging speeds.