Both the parent & child are custom themes I’ve written with my team. We have 15+ other child themes inheriting from the same parent, and we’ve never seen a template file ignored like this before.
I wish I could help, but I’m having the same problem. If I use any user name or any other string or hash tag for search, the tweets are received correctly. But the one string I need to use doesn’t work. It isn’t a character limit issue. I’m getting an error message “No Tweet results for search ” with a link to the search… but that links to Twitter where there ARE tweet results for that string.