That about covers it. It is not set for Silent. I did change the default tone. But I don't think it was making any noise/vib before.
Holy crap! It may involve all of those things. Yes, there's a Windows' Messages running. My Google Watch is also there. I will go over your list and see if I can narrow it down... I should add that I did not have this problem with the Original Pixel Fold. I heard alerts and all else was probably the same.If this is for Google Messages, does this happen while you're using the phone or all the time? If you're using the phone I think Messages just displays the notification; doesn't vibrate/sound it out.
Also, are you logged into any other session (i.e. Web portal)? Messages is notorious for messing up notifications with multiple sessions open, so another location may be getting the 'full' notification while the rest get only pop ups (or none at all).
Have you checked the category section to make sure that the conversations you're seeing aren't individually set to a different ringtone or silent?
Are you using any smartwatches or notification apps that may be 'intercepting' the notifications?