Due to limited server capacity, currently when selecting to delete messages, a maximum of 30 messages can appear at once, so if when  you choose to delete all,  30 messages will be deleted, if there are more than 30 messages, you may need repeat the delete opreation multiple times.

The server is set to wipe out the messages after 30 days, if you do not want to speed too much time deleting the messages, you can simply leave them there and they will be erased after 30 days.