Take a look at https://deno.com/wp-content - unsurprisingly it will yield a 404. However, https://deno.com/should-not-exist will yield a different 404. The same applies for https://usefresh.dev/wp-content and presumably for any other website hosted on Deno Deploy.
Does anybody have an idea, why or when this bug might have been introduced?
When I looked the other day, Deno Deploy was hosting 46 websites, I tried a few but they all worked so I didn't land on your site.
As for a silent response, I've seen similar trying to mirror where what I'm using doesn't have the correct or necessary permissions or settings.
I'd suggest you contact Deno Deploy to find out why your redirect isn't working.
It's a common attack vector. Lots of scanners look for it.