Ironhelm combo resets as well as thrust stacks reset on attempting H2 then pressing sprint to gain extra distance. this also happens on crusher but the draw time is far lower so its less of an issue.
I like this “feature” personally as it adds nice skill based tech to the weapon that makes its horde clear combo faster. If you know it exists, its easily played around. Ironhelm isnt the only weapon that can sprint cancel either.