The core issue, I think, is that Apple has added some sort of iOS-only enforcement in which third-party app Quick Look extensions cannot be invoked when the file type is a system-provided UTI.
Since iOS and macOS 27 introduced the Markdown UTI as a built-in type, third-party extensions have stopped working on iOS even though the system does not really provide a Quick Look extension for net.daringfireball.markdown and falls back to the public.text preview.
Apple should remove that limitation and align the iOS behavior with macOS, in which if a third-party extension can preview a more specific type, it should be the one producing the preview (not a general system fallback extension).
Feedback filed as FB24481377.
Topic:
App & System Services
SubTopic:
General
Tags: