I've seen numerous bug reports relating to issues with the Add Boolean function so this may well be something that the team are already aware off (apologies if that is the case), though Matt mentioned in March 2020 that "Boolean operations between two objects where one of them was rotated were already known and fixed" in 1.82.
I'm still seeing an issue with rotated objects (specifically rectangles, i.e. the same is not true following the same procedure when using the ellipse tool)) and the Add Boolean operation which exists in 1.7.3 and 1.8.3. I've attached a short screen recording. When the second rectangle is rotated +90˚ the add function doesn't give the correct result when the Add Boolean is applied, yet when rotated -90˚ all works as expected. Converting both rectangles to curves prior to applying the Boolean Add operation makes no difference and neither does reversing the direction of the curve on the second rectangle after rotating +90˚.
Boolean_Add_Operation.mp4
Add Boolean.afdesign