I have had similar experiences, and I think there is a bug (or Perk/Feature like some have suggested) where arrows ignore all armor in certain situations.
The first time I experienced this was with a Drougr Deathlord, in that one room with all the traps on the floor where a spike pops up if you stand on it too long. Can’t remember the name of that specific dungeon off the top of my head…
Anyway, while I was busy taking care of his minion, this guy would shoot arrows at me, and between dodging the spikes and fighting the other Drougr, one hit me. My armor is over 400 at this point, and I had the perk that causes arrows which hit my shield to do no damage.
Seeing he was lining up a shot I raised my shield, and the arrow hit said shield, and therefore should have done no damage. However, it instead took me down to almost zero health.
I thought it was a gimmick thing with that boss having some special bow/arrow (which couldn’t be looted), until I fought through Forelhost Stronghold (awesome dungeon BTW), now with over 600 armor at level 42. There were a couple Drougr’s with Ebony bows taking shots at me and again, one shot even with my shield up would reliably take me down to under 100 health.
I’m actually not complaining though. It forces mele to think more about their surroundings, kite around corners (and into traps; I wasted two Drougr Deathlords at once on a swinging gate!), and actually makes potions like Fortify Block/Magic resist and Heavy Armor as well as Regenerate Health/Stamina a lot more useful than in previous games (I hardly EVER used potions in Oblivion.)
Plus it forces you to work shouts into the rotation more regularly, and Shield Bash to interrupt/Disarm. Having a lot of fun with it!

EDIT: I do play on Master mode... Might have something to do with it.