Based on what you've described, it sounds like ProGet is indeed blocking downloads; this is visible in the ProGet Web UI with a "Download Blocked" indicator. If you try accessing the download URL, you will in fact get a 400 error.
However, NuGet/Visual Studio aggressively cache package - which means they aren't even attempting to download them. If you clear all the NuGet caches (system, user, http, project, etc), then it should attempt to download then again.
That said, as of ProGet 2026, we no longer recommend downloads. This is one reason, but there are more reasons.
Here's an work-in-progress article that discusses our new guidance:
https://guides.inedo.com/vulnerability-management/containment/
Cheers,
Alana
