Google is now deciding some already-published comments are spam

Something weird is going on where Google will retroactively decide that some comment that has already been published here and has been visible for days is actually spam, and then it disappears. Sometimes this even happens with signed-in comments by me, the author of the blog! I just discovered this today, when I found that the comment count on some of my recent posts was going down. I've manually restored all the spammed comments (100% of which were real comments from regular readers, not spam) and will try to figure out how to prevent shenanigans of this kind in the future.