WPO bug with Material Layers

I find that there are a couple of things that influence the artefact

If WPO in M_BlendingBug is connected I get an issue with the blending.
Removing the opacity input that's passed through the material attributes in MLB_Opacity removes the bug.
If I use a texture or scalar directly in MLB_Opacity the mateiral works as expected.
If I use a different material layer blend that doesn't rely on the opacity mask I don't get the issue even without removing WPO.

While trying to understand where the issue was coming from I tried using material attributes other than the opacity/ opacity mask to pass the info from the A channel in the Diffuse and with some I managed to remove the bug in some cases, but it wasn't stable (I'd move a node, hit apply and the bug would be back sort of scenario). It didn't work every time as well.

I haven't seen a similar artefact outside of this setup with Substrate.