I still think it is forge. It is a known difference with forge and my forge server also has this issue where I can't open chests till I remove the slabs on top of them from vanilla worlds where I did that. Seems to me like the guy above is the exception, but I only have my own experience to speak of.
Well, here is another experience: I've used Forge on countless worlds, dozens of different modpacks. I've never had this problem.
In particular, I've played pretty much every "skyblock" type of thing, and putting slabs above a chest is one of the very first things I do - with no torches, it's all about having no spawnable spaces at all.
Thousands of people play on Forge - If this bug happened to all of them, don't you think it'd be wider-recognized?
Just do the 'usual' Occam's Razor, binary search thing; remove half the mods, see if it's fixed.