Shader gets null matrix by default while the inspector shows it should be identity

[Zylann]

Godot 3.2.3
Windows 10 64 bits
OpenGL ES 3.0 Renderer: GeForce GTX 1060 6GB/PCIe/SSE2

I wrote a custom shader in my project which involves a custom matrix in one of its uniforms, but all the material previews using that shader are black.
I tracked this down to a matrix that ended up being filled with zeroes, even though the inspector shows it should be identity:

The bug goes away as soon as I modify the matrix manually to be slightly different than default, then it keeps working if I set it back to default. If the value is reverted from the .tres (for example as a result of "properties == default values not being saved"), the problem comes back.

Reproduction project:
BlackMaterial.zip

[lawnjelly]

Wild guess, the default is probably identity on the client side, but unset (zero) in the visual server. The client doesn't update the server when it is default, so doesn't update until it is changed. Then when it is set back to identity it works, because the update happens. There have been several bugs of this type.

Can't check this until back home. If so the solution we usually used was to set a proper default in visual server side (or alternatively you could force setting even the default value).