I don't know if this is the issue Russ is having or not, but I thought I would post here (I've also emailed Russ). I experienced similar drop-off this week with an app of mine which has been in the App Store for several years. I wouldn't have noticed except I checked after reading Russ's post.

My app has space-separated keywords in the Appstore Connect portal. They've always been space-separated and I can't remember the last time I updated them. But the App Store guidance says to comma-separate keywords. Indeed, the newer apps I work on for my day job have comma-separated keywords. Could Apple have changed how keywords are parsed, and broken older keyword sets that used to be valid but now are not?

I'll be updating my app tonight and the keywords along with it; time will tell if this is the culprit.