Transform Rules not being applied/recognized

[cinder thorn]

I have a transform rule to rewrite the URL from, as an example, https://example.com/subtitles/12345.vtt into https://example2.com/file/12345.vtt
I want to preserve the path from the original request but load the file from a different domain.
It seems that I can't even get the rule to trigger (regardless of the query) and I have checked there are no conflicting Workers or other rules.

The rule I have at the moment is
(starts_with(http.request.uri.path, "subtitles"))

If I do manage to trigger the rule, how would I extract the end part of the file extension and put this into the rewrite url?
I was able to do this using Workers but not with Transform rules (I want to avoid being restricted by the 100,000 limit if I can)

[red snow]

A path will never not start a /, try swapping it out for "/subtitles"

[cinder thorn]

i've tried that too, but it still takes me to the react app instead of the static file. I've also tried (http.request.full_uri contains ".vtt") but that doesn't seem to trigger either

[red snow]

by any chance is Automatic Signed Exchanges enabled in the zone?
if not, can you share a screenshot of the transform rule?

[cinder thorn]

concat("https://f004.backblazeb2.com/file/mybucket/", http.request.uri.path)

Automatic signed exchanges is not enabled

[red snow]

thanks