Take this v2 query:

location.country: "United Sta???" or location.country: "In???"

This produces 52.58M “United States” and 4.56M “India” results.

When passed into the v3 query converter it produces:

host.location.country:{"^United Sta...$", "^In...$"}

Searching with this in v3 shows no results.

However, this does produce results (52M “United States”, 4.3M “India”)

host.location.country:{"^United States$", "^India$"}

It seems like using regex within bracket notation is not fully supported (like only the ^ and $ anchors work here). If this is the case, the query converter should not be returning this query. Perhaps instead it should do the regex long form, which does work:
 

host.location.country =~ "^United Sta...$" or host.location.country =~ "^In...$"

We have a proprietary v2 query similar to this form, but we cannot use the converted query as returned.