https://bugs.winehq.org/show_bug.cgi?id=56205
--- Comment #2 from Zeb Figura z.figura12@gmail.com --- It does a colour fill of a 32-bit RGB sysmem surface without alpha. The colour fill has alpha == 0. It then locks this surface and creates a DC from it. The game apparently expects this alpha to be retained even though the surface doesn't actually have an alpha format.
Testing shows that this is the case on modern Windows + WARP, but that's not very meaningful. It needs testing on older hardware and other drivers. I'm working on that.