What is the version of your Exchange server? Different version of Exchange have different OWA layout, which means user need to change password from different places.
Do you mean change password in OWA rather than ECP? Could you provide screenshot about good and bad mailbox? It will help us to narrow down this issue.
If the response is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.