OK I think I’ve identified the issue. And it is specific to Vercel.
Apparently Vercel tries to be helpful by automatically parsing incoming function requests. See these:
https://vercel.com/blog/vercel-node-helpers
https://vercel.com/docs/runtimes
Per the end of that latter doc, you can disable these helpers by setting NODEJS_HELPERS=0. Did this in staging and now pagination is fixed 
So I guess our express app (or the forest client?) sees the request params are already parsed and doesn’t bother. And Vercel’s helper doesn’t handle nested query params.
Anyways, thanks for the help guys. This was a tough one 