I tried to add a custom web search for a locally stored copy of the Godot Documentation (https://docs.godotengine.org/en/stable/ - Scroll down to Offline Documentation and download Stable, then open index.html)
It offers a very simple but useful javascript based search that works locally: file:///Users/gruni/Downloads/godot-docs-html-stable/search.html?q={query}&check_keywords=yes&area=default# with + (plus) as space character. The search does not only work when triggered from the input field - but also when opening the file directly with these parameters, so I thought great: I'll create a custom websearch for it and can save some time.
However, when actually triggering the search with one or multiple keywords, Alfred only opens the web browser straight to file:///Users/gruni/Downloads/godot-docs-html-stable/search.html without any additional parameters - unlike with any other one of my custom web searches set up using the http(s) protocol.
So I assume it's a problem directly tied to using file:// with Alfred itself.
Replicable with Alfred 5.5 (2257) on macOS 14.5-7 and 15.0 regardless of the web browser (tested with Firefox, Safari, Edge, Opera and Orion set as default browser, just in case)