A dynamic mark below the staff must clear the stem/notehead it is attached to

A dynamic letter sits below the staff at a fixed minimum distance from the bottom line, but must also clear any stem that projects down toward it from a low note.

A dynamic mark under a very low note (stem-down tip projecting well below the staff) overlaps the top of the dynamic letter's bounding box.

Gould, Behind Bars, ch. 'Dynamics'; Ross, The Art of Music Engraving dynamics placement section.

This rule is about where marks land on the page, so it can only be judged after engraving. Collision and spacing faults live here.