Saw this in the Xisumavoid video, he listed it as an intentional change. If so, it's worked around extremely easily.

Here's the tests done:
Bow+Mending=Mending Bow
Bow+Infinity=Infinity Bow

Infinity Bow+Mending=Infinity+Mending Bow
Infinity Bow+Mending Bow= Infinity+Mending Bow

Mending Bow+Infinity Bow = Mending Bow
Mending Bow+Infinity = Mending Bow
Mending Bow+(Power/Unbreaking/Binding/Vanishing/Flame/Punch)= Mending+Other Enchantment Bow
Mending+Other Enchantment Bow+Infinity=Mending+Other Enchantment Bow
Mending+Other Enchantment Bow+Infinity Bow=Mending+Other Enchantment Bow

In conclusion, Mending specifically excludes Infinity if it is present in the First slot of the Anvil, but will allow the combination if it is the second slot. Easily worked around if this is intentional, definitely a bug if not intentional.

(First Bug report, hope it's formatted effectively)