Why doesn't ignite proc Dark Harvest?

It used too, was deemed to problematic, now it doesn't

directly from the patch notes when it was removed:

Dark Harvest Base damage decreased. Can't proc off other procs.

We followed up 8.23 with a few quick nerfs to Dark Harvest's damage, and 8.24 brings another. We're shaving off baseline strength, particularly in the early game, to ensure Dark Harvest's performance relies on how effectively its users can stack it up. On that note, we're also making it harder to stack by removing its proc-off-proc cases, which actually makes the premise a bit more clear: If your target's below the threshold before you hit them, you'll get a stack.

BASE DAMAGE: 40-80 (lv 1-18) ⇒ 20-60 (lv 1-18)

PROC MODEL: Can no longer proc off other procs (ex. Scorch, Liandry's Torment) aside from Luden's Echo, whose effect is built to behave like a separate ability