I have an Arsenal AK (not sure of model number, its in my safe) that I bought 4 or 5 years ago. Plain ak-47, synthetic furniture.

Problem is it shoots hard left, and it never did that before a few weeks ago. It wasn't just me, it was a friend of mine two who was experiencing this.
It's not been dropped or knocked around.

Tula Ammo, FMJ was what we were using, I didn't have any other ammo to try out but it was a good 5-6" left at 50 yards

Any suggestions on what to check? I don't mind getting a rail system and putting a red dot on it, but I'd rather have the iron sights work properly

Weird one.

I'd check the muzzle attachment to insure it's screwed on all the way to the locking notch and you're not getting bullet strikes on one edge.
All AK's will have slightly loose muzzle attachments, but they tighten up after part of a magazine fouls them.
Altering the attachment to get them totally tight serves no purpose and may make the attachment difficult to get off at all after some rounds have been fired.

Check for loose or canted sights or barrel.  If something is loose the barrel or barrel parts may be able to shift.
Check for a loose front sight drum that may have moved.

I know you said it hasn't been knocked around but look anyway for canted sights or other damage.

Did you change ammo? I had an issue years back with switching between Brass cased Cheetah from Africa somewhere, and Steel cased Russian pre Wolf era. POI shift was ~5" right with the Steel cased at 100yds. If your ammo hasn't changed I'd check to make sure your FSB is pinned properly and not loose. Also check your rear sight to make sure it's tight. Pulled a TWS top cover of a VEPR and unsprung the sight spring -- it would shift left and right if pushed when I put the dust cover and rear sight back on.

Something changed. If it were the ammo alone it would account for maybe 2" at 100 yards, but 6" at 50 yds is a major change. See if the front sight base is loose. See if the rear sight leaf or base is loose. Check for a loose or bent barrel.