Hi Mark, probably related to these 2 lines here being apart by such a small distance kind of skimming by each other:



Zooming in there, those lines are apart from each other by a distance of 0.0006
units:



This kind of situation with pieces skimming by each other by such a small amount can make things a whole lot more difficult for the booleans to make sense of, it's a lot better for things to either be exactly aligned with one another or to be pushing through each other by a somewhat larger distance than that.

- Michael