That breakdown is interesting.
There are 4 power pages in ANT+. Basic (everyone, mandatory), Crank Torque (cranks, arms, pedals), Wheel Torque (Powertap hub only) and Crank Torque Frequency (SRM only). Everyone must have at a minimum basic pages, and almost all use 1 of the 3 others with every 4th page is basic to maintain backwards compatibility with simpler head units -- least that was the idea. Basic pages are also the only place balance is found. It's a violation of ANT+ spec to include multiple option pages and it wouldn't have certified. So 1 + basic.
So based on what has been shown so far I would guess that the Shimano uses Crank Torque pages. Since SRM only has to deal with Crank Torque Frequency in their own products, when connecting another meter it's likely dropping back to Basic pages. I don't have a PC8 so can't check.
The possibility is that either the Shimano PM is not encoding basic pages properly or the SRM head unit is not decoding them correctly.
Also, when testing, what a Garmin records is NOT what it displays. They have different requirements. The specific example the Garmin Edge engineer used at the ANT+ symposium a few years ago when I was there was to imagine going down a hill and you stop pedaling, when you look down you want to see what your cadence was not zero -- so it holds it for a few seconds even though it knows the data is stagnant and not changing. What is displayed in all Garmin head units is filtered, what is recorded on the newer (510/810 and newer) follows good guidelines for proper recording.
These spikes should be easy to track down using the ANT+ development software because it can decode both pages unlike a head unit. My guess is the PM and not the SRM head unit. Likely escaped because nobody saw it in crank torque pages.
kwakekeham, I licensed the ANT+ Adopter SDK MacOS to log the raw ANT+ data stream from my FC-R9100-P, and now have more information concerning the 64sec power spike issue with my SRM PC8. My plan is to document and forward my analysis to Shimano and SRM within a few days.
FYI, FC-R9100-P acts as an ANT+ Power-Only Sensor, and exclusively uses the ANT+ Standard Power-Only Main Data Page at ~2Hz to encode power, cadence, and pedal contributions.
FC-R9100-P also transmits the required Common Data Pages and several Optional Data Pages.