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I'm leaning towards it being a time- and materials-efficient way to fill the gap. Bricks were recycled from kitchen extension that was coming down. The new extension will have one brick wall but the rest will be wood. I've raised the finish with the architect, who's not worried. But I'll certainly raise it with the builder and just check that he's aware that this sort of finish on exposed brickwork would be a very big problem. I don't think that would ever happen, but for peace of mind I'll raise it.
I think there's also some variables/context which would influence my view.
Eg If a brickie who's doing the rest of the extention did it, maybe it's a bit worrying. But if the brickie isn't on site until next week or whatever, given it's not being seen it would be inefficient to either get one on site to fill a hole like that, or not make use of the resources already on site to tick the job off.
In which case if a labourer has been told to get it filled by the end of the day, then it seems fair enough to me.
Contacting the architect is a good idea anyway, because at least it sets a bit of a tone that you care about this sort of stuff.