There are approximately 3000 or more users who have the same first and last names in our tenant.
For most of these users, the only difference in the usernames are either a period or an underscore that separates their first and last names.
If the username's period is replaced with an underscore, it will override someone else's profile picture and this is creating a big problem.
Please do not replace the period with an underscore when the photos get stored in the folder User Photos document library.
https://support.microsoft.com/ja-jp/help/3185286/information-about-profile-picture-synchronization-in-office-365