Search results webpart shows placeholders instead of user profile pictures

Hi,

we have a search results webpart that, among other properties, displays user profile pictures according to the ootb "Local People Results" result source.

Unfortunately we're seeing more placeholders lately instead of the actual photos for users whose pictures had showed properly.

I can see that the images are pulled from "-my.sharepoint.com/User%20Photos/".

In this library every user that has ever existed in the tenant has stored profile pictures in two or three different stylings:

- _LThumb.jpg

- _SThumb.jpg

- _MThumb.jpg

I noticed that for some users all of those are indeed placeholders. On the other hand, looking at an affected user's profile in the Admin Center it shows the correct photo, not a placeholder.

My question is: Based on the fact that the user profile shows a correct photo but
"-my.sharepoint.com/User%20Photos/" may contain placeholders, what is happening behind the scenes that could cause these changes?

Does anybody know what generates those two or three different images per user in the "-my.sharepoint.com/User%20Photos/" library?

Thanks for your help