The midtexture grating here is uneven.

Fixed

It still is.

In the DB preview it's fine?!

DB has a tendency to screw up the offsets for midtextures in UDMF.

Okay, this is one of the more bizarre things I've seen. ZDoom is ignoring the offsets here. I've corrected them in two different ways and the texture alignment literally will not change. I have no idea what is wrong here, aside from a possible engine bug.

Well, it's mostly offset-ed properly but not quite (you can see in the screenshot that it's still 1 pixel/unit off). On top of that there is this midtexture bleeding-like issue, which is odd, since the clipmidtextures flag is already set in MAPINFO.

Bleh. I dunno what to chalk this one up to now. Definitely an engine "whoops" since the clipmidtexture flag is set explicitly, too.

This is actually very simple to explain - slopes aren't taken into account when clipping mid-textures and never have been. you either live with it or move the grate out of the slopes.

Or add a sector in between that isn't sloped but 2px thick, and don't add any lower textures