This is a known bug. As a workaround do the following:
For each host, go to the "Configuration" tab
In the "Software" section, click on the "System Resource Allocation" link.
Click on the "Advanced" link on the top right.
Navigate to the host/vim/vmvisor resource pool
Click "Edit settings..."
In the "Memory Resources" section, set the limit to the same as the reservation.
Elisha