It seems to have worked! If I delete my post → reset bump date (which seems to clear the ‘Solved’ checkbox from my post) → undelete my post → click ‘Solved’ again on my post → then it no longer appears at the top of the forum list 
It seems to have worked! If I delete my post → reset bump date (which seems to clear the ‘Solved’ checkbox from my post) → undelete my post → click ‘Solved’ again on my post → then it no longer appears at the top of the forum list