Thunderbird client not deleting emails from Cox IMAP server

I am using Thunderbird v68.5.0 on Windows 10. I have configured it to the Cox IMAP server and successfully get my email. However when I delete an email in the Thunderbird client (even from the Trash folder) it does not delete it from the IMAP server. I confirm this by logging into the Cox web-based email app and see the message grayed out with a line through it (indicating it has been deleted) - but my Mail Quota did not go down. The message I deleted had a 500Mb attachment. If I delete it this message in the Cox Email app it is moved to the Trash folder (but not deleted from the server, as expected). If I delete it from the Trash folder in the Cox Email app, it is then removed from the server and my Mail Quota goes down. I thought IMAP synced with the server from all clients and deletion of a message from any of the clients would delete the message from the IMAP server.

I also tested moving the message in Thunderbird client into Thunderbird's  Inbox under "Local Folders" - I had read that moving a message into the Local Folders was equivalent to deleting the message as far as the IMAP server was concerned - this did *not* delete the message from the IMAP server (as verified with the Mail Quota readout).

I also tried following a posting from about 4 years ago in this forum that said to go into the Thunderbird menu Options->Advanced->Config Editor and set the mail.imap.expunge_after_delete option to true, restart Thunderbird, and then messages deleted in Thunderbird should be deleted from the IMAP server - this too did not work.

So I am confused (and lost) - what am I doing wrong? In Thunderbird I have my mail account's Server Settings as follows for deleting a message:

"When I delete a message:

      (selected) Move it to this folder: Trash on <my mail account name>

      (unselected) Just mark it as deleted

      (unselected) Remove it immediately

I want Thunderbird to move it to a Trash folder and then I delete it (like the Cox Email app works).

BTW, I actually configured a message filter in Thunderbird for all my Cox email accounts to move all incoming messages for all mail accounts to the Local Folders\Inbox folder (similar to how I used to have it when I used POP3 across multiple mail accounts. With this message filter I would have assumed all incoming messages should have been deleted when they were moved to the Local Folders\Inbox, but they were not. But for purposes of further debugging this whole issue I disabled this message filter. But am curious if this message filter should behave like I expected.

Thanks for all the help anyone can provide.

Hugh