TYPO3 Core

Yes recipients are configured for the custom stage
and pop up dialog box is shown

edit :
I've just change this in "Recipient suggestion checkboxes" :
before the choice was :All are selected per default (can be changed)

now I've choose : All are selected per default (cannot be changed)

And it's seems to send email notification ...

I've got the same Problem (TYPO3 4.7.12)

The problem is, that if you choose "All are selected per default (cannot be changed)", you get a list of checkboxes, which are checked, but disabled.
Browsers (at least Chrome) do not send the value of disabled form fields. So if you submit the form, the values of the checkboxes are not sent.

The function Tx_Workspaces_ExtDirect_ActionHandler::getRecipientList should get a list of backend users which should be notified. It tries to do that on line 248:

245:    if (intval($notification_mode) === Tx_Workspaces_Service_Stages::MODE_NOTIFY_ALL || intval($notification_mode) === Tx_Workspaces_Service_Stages::MODE_NOTIFY_ALL_STRICT) {
246:        // get the default recipients from the stage configuration
247:        // the default recipients needs to be added in some cases of the notification_mode
248:        $default_recipients = $this->getStageService()->getResponsibleBeUser($stageId, true);

If I change the true to false, it works for me. I do not know if there are other consequences or if this is the correct fix for this bug.