Version: 12.1.0-beta-2

https://www.youtube.com/watch?v=a4ASJeH1N4w

Seems like a valid structure only forms if the blocks are placed in the right order. There is probably a lot more examples and edge cases but this issue seems to be at least one that's at the bottom of it.

I haven't ever seen this in my own testing.

Might have been fixed by the other commits related to fireboxes (?)

Still happening for me in beta 6, could it be that no block updates get send to the firebox? (just a random guess)
Wonder if another mod is causing this if you cannot reproduce in your dev environment.

What about chunk boundaries?

But yeah, it looks like a block change propagation issue.

What about chunk boundaries?

Tested with some multiblock configurations that do not cross any chunk borders at all in beta 7 just now.
From what I can tell any boiler multiblock only starts looking for a valid structure when a block of either of the bottom 2 layers is placed. Any boiler tanks placed above other boiler tanks instead of the fireboxes means no multiblock forms even if the structure is valid.

I'll do some more tests when I next get a chance.

Okay, I have confirmed the issue.