friends and i were raiding a fortress with similar netherite armor with projectile protection 4  fire protection 4 and thorns 3 and thorns 2 and mending x 4 and unbreaking 3 x4 and  a week erlyer wither skeletons  did 2 hearts and 1 to 2 for the wither effect but now when raiding a fortress a week later the wither skeletons did 8 to 9 hearts of damage and the rest of our health gone to the wither effect . and sometimes it does  only 2 hearts and 1 to 2 for wither effect and others it does all heath from one hit and the wither effect