I started recently to work on a spare part to print with a 3d-printer and I repeated my workflow that was successful in prior versions of blender and then i noticed changes in the with the bools.
I can not find apply for the boolean operation since i upgraded to 2.9 (from 2.83)
The first picture is the bool operations for blender 2.9,
The second picture is the same file in blender 2.83.
