I have a mostly empty 7x7x8 iron tank with lava in it. A enderman teleported inside the tank and is just fine and dandy living inside.

I have a video of it sitting in there, I don't know if there is anything I can give other than that for reporting wise. I guess I can give you the world save if you want it.

I am using the FTB Direwolf20 v4 modpack with no mods added or removed nor any special configuration.

Not much I can do that won't cause lag or be needlessly complicated.

Couldn't you just make the internal tank solid with a transparent block that appear when the tank is finished and is removed when the tank is broken? Or would that mess up a lot of other parts of the coding for the tank gfx?

P.S. Did I catch you checking the project or are you just superman with bug reports all the time. I am very impressed!

That would fall into the "needlessly complicated" category. ;)