During our investigation we found out, that administative users, guests and users without Exchange license are not affected by this issue. Also there is no any connection between "digits instead of name" problem to version of Teams (new or classic),
no relation to license (Teams free, Microsoft Business 365, E3, and so on), nither it is connected to cache cleaning. All you need is just to click on one single checkmark
in the user setting in Microsoft 365 admin center (aka Microsoft Portal).