what has caused my apple watch 8 to intermittently ping while it is charging?

However, you state that you are using the puck that came with the watch, not a 3rd party charger.

I've never encountered this before and am at a loss.

Rather than the charging puck, the wall adapter you are using may be the issue. Is it an Apple charger, or 3rd-party?

M1 Mac mini, macOS 26;  Watch, Series 10 GPS-only, watchOS 26; iPhone 16, iOS 26; TV 4th Gen, tvOS 26 HomePods, audioOS 26

This is pretty common when using 3rd party chargers. It seems to come and go with watchOS updates. It's not really a problem, just annoying.