Although I have not seen messages showing up in Spam, just this morning one of my co-moderators said that he does. He is seeing a sudden increase in group messages that went into his Spam folder in Gmail.
It could be that Google/Gmail is phasing in changes to their algorithms, and the changes reached J and my co-moderator, but not me yet. Google does that kind of thing. It's happened before.
The co-moderator also wonders whether that hijacker who targets AOL addresses may be partly to blame. My group gets hit fairly hard by that idiot. If a lot of AOL users who are not subscribers to our group receive group messages and mark them as Spam, might it somehow trickle down to other email providers (such as Gmail) and cause Gmail to mark legitimate messages as Spam too, just because they came from our group or from groups.io? (Not talking about blacklisting.)
The hijacker went on hiatus for a month but it seems he returned about Nov. 6.