Wow, I... didn't know that gesture was a thing. Thank you! It seems to help a bit! I'm now getting a 50/50 son/soon ratio. That said: when I manually type "soon" -- definitely with a double "o" -- it still autocorrects to "son" on the first try. So, going to keep my pet theory intact.

The weird thing is that if I type nothing at all, the contextual predicted "next word" on GBoard is actually very good -- I wasn't praising it for comedic effect. But it really does seem like there's a sign error in a sort function which kicks in after you start typing.