AD beta 1.8.3.1
macOS 10.13.6
A faulty Boolean union results from the pair of closed Curve objects in this example: faulty add.afdesign
Reversing the stack order of the objects enables a correct union to be made.
Also, scaling down the objects by, for example, 50% enables a correct union to be made.
The node I've highlighted in the 'before' screenshot appears to be significant. Nudging it out of collinearity with its neighbours, or deleting it, enables a correct union to be made without scaling or stack inversion.