Revelations from Patch 6.5: "Growing Light", Part 1

[Cleretic]

You seem to be conflating two things here, though (although perhaps Grimr is too). You're talking about the wards that imprisoned Zodiark within the Source – I would suggest that they never tried to break Zodiark out of that because it may also act as a sort of life-support system keeping Zodiark in stasis but alive while he is broken into pieces.

On the other hand, what Zeromus was trying to do was tear open a hole between dimensions, which might be achieved without ever physically breaking Zodiark's wards. (Like a higher-dimensional version of a prison that stops you from moving up and down, forward and back, but you can just step out of it "sideways" – into a different shard in this case.)

No, Grindr and myself were both talking about the wards, which are why the moon gate was opened in the Zodiark Hole in the first place. Those were exactly what was straining against Zeromus as an established concern and timer; the very first scene of 6.5 reiterates this one, and I think it's Vrtra that puts it most succinctly:

Yet though these measures were sufficient to imprison a fragment of Zodiark for eons, against our enemy's determined onslaught, they are degrading at an unsettling rate.

It's not the Thirteenth's wards that it's trying to bust through: it's the Source's, through the gate we made.

I'm pretty sure that's what Grindr was complaining about, because it matches his complaint, although as we've both said it completely makes sense as to why this is happening: Zodiark was a dormant and crippled prisoner, while Zeromus is whole and very much shaking the bars.

It's only once we go down and challenge Zeromus that she actually starts cracking some holes in the wall between shards directly: that kinda seems to take everyone by surprise, possibly including Zeromus herself (if she were cognizant enough to notice), and doesn't seem to be the central crux of the threat to anyone, but in retrospect isn't exactly out of nowhere; she's just making bigger versions of the fractures that most non-contracted voidsent naturally slip through.