"This package lacks a signature." warning is thrown when installing Editor Coroutines package

Reproduction steps:
1. Create a new 3D project
2. Open the Package Manager window (Window > Package Management > Package Manager)
3. Install the “Editor Coroutines” package
4. Observe the output

Expected result: Package is installed successfully
Actual result: Package throws a “This package lacks a signature. Select Read More for information about signed and unsigned packages.” warning

Reproducible with: 6000.3.0a4, 6000.3.0a6
Not reproducible with: 6000.0.56f1, 6000.2.2f1, 6000.3.0a3

Reproducible on: Ubuntu 25.04, Windows 11
Not reproducible on: No other environments tested