Found the problem. Was having a problem with the deadlock integration mod and had it disabled. Was too impatient to wait for the author to update it, so I tackled it. Once I got that mod running, glass magically was stackable. No idea why, wasn't even the reason I was fixing the integration mod....but i'm not complaining.
Can't really do much about this, It's fine as far as I could test it with SE deadlock bridge for 2.0 only, likely Deadlock integrations/compatibility for AB+ breaks something, if that's the mod you're referring to. That mod, by description deletes every previously created stacking recipe.
