System Information
Operating system: Windows-10-10.0.18362-SP0 64 Bits
Graphics card: GeForce GTX 1070/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 460.89

Blender Version
Broken: version: 2.93.0 Alpha, branch: master, commit date: 2021-02-25 05:34, hash: rB0e1c6a29cbb2
Worked: (newest version of Blender that worked as expected)

Short description of error

When creating a group and giving one of the group parameters a default value, the default value is not used when assigning the material to a new object, or when right clicking the parameter and selecting to reset to default value.

Perhaps new materials are not supposed to initialise contained groups to their default values? In which case it's just the reset to default value that appears faulty.