Teams Feed shows first name of people

Hello Community!

In our teams feed it only shows the first name of people. In the past it was the last name. We didn't change anything in our settings - so what can be the reason?

I found following discussion with similar problem: 

Re: Teams Feed shows last name of people - Microsoft Tech Community

But it doesn't help, because our display name settings are: Lastname, Firstname - so why is the second part of attribute shown?

Thank you in advance!

BR

Joanna