Cannot change priority of dcp connection

I tried changing priority of DCP connection as follows:
1. Set priority when DCP connection was set up. – no error here.
2. Set/change priority after DCP connection was set up.

The following error was seen in memcached log at step 2.

2018-12-04T10:55:00.239768Z WARNING 512096080: cookie_get_priority: priority (which is 16) is not a valid CONN_PRIORITY - closing connection (<86>^^^A@^@@@<86>^^^A@^@@à<86>^^^A@^@@<80>,<86>^^^A@^@@Ð-<86>^^^A@^@@@/<86>^^^A@^@@p

I tried setting different priorities at step 1 and step 2, and the same error message about invalid value of 16 was returned regardless. There seems to be issues with the way the dcp connection maintains priority.

cbcollect attached.