I posted a fix here:
Previews in Material Nodes always black (UE5.6 Substrate Enabled) - General / Issues and Bug Reporting - Epic Developer Community Forums
Epic released a fix that’ll be in 5.7, but I found the line of code they missed from their ue5-main branch (their dev branch). Add the change to the file mentioned, and you probably need to just compile it with Visual Studio or another IDE for the binaries and you’ll be set. It should be a pretty painless mod, considering it’s a single line.