What a great post, you clearly have the “Support Engineer mentality” in specifying your experiments and results very clearly!
The docs are a bit confusing there since we’re halfway through this work: Changes to Redirects with query string parameters are coming but TL;DR the “200” redirect is more of a “rewrite” and there query string params ARE intended to be passed on as is without a specific param1=:value1 as documented here: Redirect options | Netlify Docs.
I think you’re saying that they are not, in fact, passed on - and I think that may be due to a bug around how we cache - we don’t always cache separately based on param, so you may have gotten a result from a prior param-less request when you made yours maybe? But, I am not so sure of that diagnosis. Could you let me know if your function is still deployed somewhere I can access it, to do some testing to see if I can figure things out? (I get a 404 at the presumed prod URL https://www.macosicongallery.com/api/v1/search)