Seems that the vSphere Web Client has some bugs, also with the latest vSphere 6.5 version. It should be the main web client (see a list of possible GUI clients in vSphere 6.5), but sometimes does not work as expected (without considering possible Flash bugs). If you are using vSphere vApp (a feature that requires a DRS enabled cluster), you may have some issues when you need to remove a VM outside from a vApp.