DRS does not evacuate a host requested to enter maintenance mode or standby mode.
Problem
When you attempt to put a host into maintenance or standby mode, DRS does not evacuate the host as expected.
Cause
vSphere HA is activated and evacuating this host might violate HA failover capacity.
Solution
There is no solution. If appropriate, deactivate vSphere HA before you attempt to put the host into maintenance mode or standby mode.