The currently latest PAC information (Hellfire Gala card) about how size affects breaking away uses the dice images to state that characters a step larger break away on 3-6.

Plasticity still states that unless you meet certain criteria, you break away on a 6.

Which takes precedence?

(NOTE: Before now, it was worded that being larger gave a +1 to breaking away, NOT a break on a flat roll of 3-6.)

In the past plasticity had the expectation written in it that it did affect the character of larger size.
However, as written currently my read is tyha6 any character not possessing the powers listed by spasticity breaks away on a 6 regardless of size

From the Comprehensive Rulebook (this was on the Empyre PAC, but removed after that):

Characters two sizes larger don't need to roll.

That isn't the problem. That text states breaking away for one step larger happens on 3-6, NOT at a +1 to the roll, presenting the problem in my initial post.

Coyote's Own claimed that all characters missing the listed powers would need to roll.

So a tiny with Plasticity now locks down Galactus.

Right.

Galactus is a Colossal, some Giant with Plasticity will lock him down.
He doesn't need to roll to break away from normal or tiny characters.

I think the assumption is that 'automatuc breakaway' > plasticity, but the big question is how are 'breakaway on 3-6' from bigger sized and 'only breaks away on 6' from plasticity calculated? I scoured the comprehensive rulebook and found no answers.

I take that bigger size changes "break away on 3-6" into "break away 4-6", and since plasticity supersedes "break away on 3-6" it also supersedes "break away 4-6".

And since automatic breakaway means you don't roll that "break away only on roll of 6".