Hello,
You code addition seems fine.
However, it is one of the customizations to the Details View in the Material Instance Editor that is causing this behavior.
The UI refresh is normally triggered by property node validation. However, the Material Instance Editor uses a custom validator, which causes the UI refresh to be missed.
The custom validator is a pass-through validator, and can be found in MaterialInstanceEditor.cpp:
void FMaterialInstanceEditor::CreateInternalWidgets() { ... auto ValidationLambda = ([](const FRootPropertyNodeList& PropertyNodeList) { return true; }); MaterialInstanceDetails->SetCustomValidatePropertyNodesFunction(FOnValidateDetailsViewPropertyNodes::CreateLambda(MoveTemp(ValidationLambda))); ... }You can try removing these two lines to workaround the issue, however, you will need to test the change. We have not thoroughly tested it.
For engine modifications like this, we can offer information about existing functionality, and we leave the details of the implementation to you.
Please let us know if this helps.