REST API PUT Removes Pool Member Description field

I'm using the REST API to enable and disable nodes.

Considering a GET to the following URL:

The following data is returned:

kind            : tm:ltm:pool:members:membersstate
name            : 1.1.l.1:443
partition       : partition
fullPath        : /partition/1.1.1.1:443
generation      : 1471
selfLink        : https://localhost/mgmt/tm/ltm/pool/~partition~pool/members/~partition~1.1.1.1:443?    ver=12.1.1
address         : 1.1.1.1
connectionLimit : 0
description     : my_description_here
dynamicRatio    : 1
ephemeral       : false
fqdn            : @{autopopulate=disabled}
inheritProfile  : enabled
logging         : disabled
monitor         : default
priorityGroup   : 0
rateLimit       : disabled
ratio           : 1
session         : monitor-enabled
state           : up

Then I update via a PUT to the same URL using the following JSON Body:

{"state":  "user-down","session":  "user-disabled"}

The proper pool member state is achieved, but I appear to lose the description field.

kind            : tm:ltm:pool:members:membersstate
name            : 1.1.l.1:443
partition       : partition
fullPath        : /partition/1.1.1.1:443
generation      : 1471
selfLink        : https://localhost/mgmt/tm/ltm/pool/~partition~pool/members/~partition~1.1.1.1:443?    ver=12.1.1
address         : 1.1.1.1
connectionLimit : 0
dynamicRatio    : 1
ephemeral       : false
fqdn            : @{autopopulate=disabled}
inheritProfile  : enabled
logging         : disabled
monitor         : default
priorityGroup   : 0
rateLimit       : disabled
ratio           : 1
session         : user-disabled
state           : user-down

I'm guessing this is a bug?