Are you by any chance using Android with App Bundle build?
If so, see this issue: Asset Bundle, not finding any files · Issue #10 · gwiazdorrr/BetterStreamingAssets · GitHub. Basically sometimes some Streaming Assets get compressed - I couldn’t find a pattern and am waiting for Unity to fix it.
Also, what Unity version are you on? Are you able to reproduce it reliably or does it change between builds?
EDIT: here’s a thread about what I’m talking about: Asset Bundles in StreamingAssets directory can load slower when building the Android App Bundle
A Unity bug report: Unity Issue Tracker - [Android] Loading assets from AssetBundles takes significantly more time when the project is built as an AAB
An interesting take: StreamingAssets files are compressed in APK when "Build App Bundle (Google Play)" option is used
So, since you already have the files in the root of StreamingAssets, try to change their name to just lowercase and see if it works.