All cylinders filling with water

From what you describe I'm getting the idea that maybe the way you're tightening the studs and the sequence you're using might actually be causing your problem. Still seems crazy but if you did both heads the same way then I guess it stands to reason.

So yeah.

Loctite=bad especially when torquing something. All you need is a minimal amount of lube. Loctice creates heat when tightened which is exactly what you don't want.

Double nuts=bad, especially when torquing. You're probably torquing the top nut into the bottom one while likely twisting a Loctited stud. You're lucky not to have broken them off in the block.

BTW, do you really need head studs? Could you just use plain old head bolts?