Steps to Reproduce:
1. Shoot a mob or player with a regular bow, using Arrows of Decay.
2. Then repeat with a bow enchanted with flame.
3. As a comparison, you can use poison arrows with a flame bow. 

Observed Results:
When using a plain bow, target takes the wither 2 damage for the 5 second duration.

When using a flame enchanted bow, target takes flame damage but does not take wither 2 damage.

But when using a poison arrow with a flame bow, target will take flame damage and poison damage simultaneously. 

Expected Results:
Expected result would be the same as with a poison arrow.  Target should take a tick of damage for flame, as well as wither 2, every second for 5 seconds.  The flame enchant on a bow, seems to negate the wither 2 effect, making those arrows useless with a flame enchanted bow.