Thanks for the headsup.
The behaviour of masked materials in 4.15 is different than in 4.14 and Alpha to Coverage is listed as supported feature here. Forward Shading Renderer in Unreal Engine | Unreal Engine 5.1 Documentation
In 4.14 there was no Anti-Aliasing at all on masked materials. And in 4.15 Alpha to Coverage is mentioned alot in the shader code. It just has the bug, of masking out too much from the texture, but the Anti-Aliasing works.