Thank You for contacting Technical Support.
After some testing and research, I have the answer to some of your questions / issues using "Private Messaging" on these forums.
Please see the information quoted below taken from https://www.phpbb.com/support/docs/en/3.1/ug/userguide/pm_new/
Composing a new message
The screen presented while composing a Private Message is nearly identical to the screen for creating a new post. The main difference is the addition of an address field for adding recipients.
Private Message Fields
Fields for adding recipients to a Private Message.
Usernames should be entered into the text area and then either the Add or Add [BCC] used to add the user to the appropriate recipient list. If you are unsure of a user's name, you can use the Find a member link to search the memberlist for the user. Place a check next to the desired user(s) in the memberlist and press Select marked to add the user to the recipient field.
Your Private Message Recipients should be displayed as shown in the example below:
If you entered a "Recipient Name" but have NOT used the "ADD" button as directed above and you click on the "Submit" button, your message is NOT sent; instead the "Submit" button works like the "ADD" button. You MUST click on the "Submit" button a 2nd time or the Private Message will not be sent (If you navigate away from the "Compose Message page" your PM will be lost forever.)