ZDoom

Bueno. Thanks, folks!

Yes, it's fine now.

I made a small change. Is it better now? I couldn't see the clipped objects anymore afterward.

Unfortunately this fix introduces some clipping issues.
I have a lot of troubles with a minimalist sample. The best thing I can do is to ask to check [url=https://www.doomworld.com/idgames/levels/doom2/Ports/s-u/sshpsos]this WAD[/url].

[attachment=0]Screenshot_Doom_20180301_170311.jpg[/attachment]
Launch with [b]-iwad doom2 -file sshpsos.zip -nomonsters +map map01 "+warp -8060 0"[/b] and rotate head to the right.
The glitch is quite common in this map, [b]warp -1070 570[/b] and turn back, armor bonuses will disappear/reappear in front of player.
Disregard missing textures, they don't matter. Also, [b]RFL_NO_CLIP_PLANES[/b] flag isn't set.

Fixed.

Alright, will tend to the 3D floor issue in the mean time. Thank you for the information.

One issue here is actually a mapping bug: To render the insides of a 3D floor you have to specify a flag for the special. For swimmable 3D floors this is always on but not for solid ones.
The other one looks like sprites which cross a clip plane aren't inserted on both sides, that one may take a bit longer to address, thanks to the insane shitload of special cases that got inserted over time.

PM me the map, I'll check but cannot promise anything.

There are two issues that I believe are strongly related: if standing still and looking at a certain angle, transparent 3D floors' bottom disappears. If there are particles in front of the 3D floor, the glitch affects them as well.

I have an example wad, but due to the privacy of the project from which I've taken certain assets to build said example, I'd like to PM the file to the dev who offers assistance.

Tested on GZDoom g3.3pre-278-ge8a7f00 (x64).