Please check if your different accounts have the same email address, if so, please delete one of the email addresses to compare the results.
In addition, here is a similar issue for your reference, the same as you, it only occurs in modern experience:
Lastly, I will suggest you to open a support ticket about it. It needs more detailed investigation with support team instead of simple troubleshooting via replies in forum. And we will appreciate it if you can update your findings here.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
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.