It’s most likely from this change:
"Re-enabled Transparent objects shadow casting which was disabled in Unreal Engine 4.26. Real time ray traced shadows of transparent objects can only be opaque. Partially translucent ray traced shadows remain limited to baked static lights. Shadow casting for transparent objects can be disabled:
- Per-material using its Details panel in the Material Editor to disable Cast Ray Traced Shadows.
- Per-object using the Level Details panel to disable Cast Shadows."
From the release notes here: Unreal Engine 4.27 Release Notes | Unreal Engine Documentation