Can't milk brown mooshrooms

Brown mooshrooms are unable to be milked with a bucket to get milk. Red mooshrooms however, can be milked.

Steps to reproduce:
1. Open any world
2. Find or summon a brown mooshroom
3. Attempt to milk it by using a bucket on the mooshroom

Expected results:
Milk will be yielded from the milking

Observed results:
Nothing happens