I have a custom mod pack and I had same issue now just crashes game. Is there a work around, I would like to use both mods. Maybe I'll post in TD bug report.
They'll probably tell you that you messed with shaders and you should piss off. Actually any mod that has something to do with Tesselator class can break those covers since their rendering is wierd and stuff and its totally noones fault but yours since you want to have fancier looking game you deviant.
But seriously now, devs could probably fix it, if it was more common problem, but they wont. But they could. I try to do something about it, but since mods I work with aren't open source I have to decompile and recompile classes which:
a) is pain in the ass when it comes to testing.
b) is probably illegal and would help deviants who want to have fancy shaders and devs would probably smite me for it so I'm only doing it for myself right now.
I wasn't able to fix those glitches from screen in first post yet but my mc won't crash anymore when someone places cover nearby when I'm on shaders. (Altho it stops whole chunks of land from rendering if cover has hole in it since td devs don't know quads. :v)