Check this wish and the two links to the corresponding threads for explanation.
It's cases like this where "but then" applies when looking for workarounds:
1. You could adjust the slab perimeter so it doesn't go above the wall (would depend on the project's design intent) but then you could have wrong slab volumes and the wall might not be shown correctly in a perpendicular section.
2. You could use a profiled beam for the tapered part of the slab, but then you'd have to keep track of two distinct elements and clean up the junction in floor plan (a case where I think fills should clean up when two elements have the same BM, like in elevations) and when exporting to IFC, for example, you'd have two elements for a single slab.
3. You could solve this if you have the walls that fall under the tapered part of the slab in a different layer with distinct intersection group and then use SEO (between wall and already SEOed slab) to cut out the remaining part of the wall.
Best thing would be if PBC would work
a SEO has been done to a certain element because SEO are element based....
other things might happen.