[ZDoom 2.6.1 ] Sprites visible behind sectors with 3D floors

[DonaldDuck]

I'm working on a remake of Witchaven for ZDoom, and I found a strange bug:

When a sector has 3d floors, sprites that are behind this affected sectors are not shown as correctly as sprites who are behind sectors without 3d floors.
Here are some images:

(Please enlarge the images)

Spoiler:

The 3D floor was supposed to simulate deep water and is attached to the floor.

[Edward-san]

why just screenshots should help developers solving the problem? demo wad or won't be fixed.

[DonaldDuck]

why just screenshots should help developers solving the problem? demo wad or won't be fixed.

EDIT:

I've tried something and now I know what causes the problem:

Spoiler:

Swimmable 3D floors cause this bug! (Type: 2)

Take this example wad.
I made 3 different sectors:

A: Sector with swimmable 3d floor.
B: Normal sector
C: Sector with solid 3d floor.

Only the sector A causes the bug. And if i turn off the swimmable option, the bug vanishes: