I have now completed Siltbreaker on all 14 heroes twice each and have no been awarded my achievement.
I note that one of my games ended weirdly (requiring me to press disconnect after clearing it while playing as witch doctor which is possibly due to the boss burrowing off the map temporarily.
Having said that, i've cleared it with witch doctor again since.

I logged a ticket with Steam support who advised me that they aren't granting achievements where the results are improperly recorded.
Unable to provide match IDs since the mentioned game was sometime ago - if I have a similar issues i will forward the MID then.