Bonestorm damage should heal for each tick of damage it does and not for each second , so now it lasts 10 seconds with max runic power and its healing 2% each second no matter the numbers of the mobs and it should be 2% for each enemy hit . So in a 10 mob Pull it should heal 20% each second.
https://www.wowhead.com/spell=194844/bonestorm
I can't find the update link of Bonestorm Talent of Legion 7.3.5 but i'l send a screenshot.
https://egammi.com/image/SyPM