Users join as a guest users to an external organization. Guest account are not shown in new Teams app. Accounts are shown in Teams Classic and in teams.microsoft.com. For some users guest accounts are shown only in teams.microsoft.com.
We have tried to clear cache files https://learn.microsoft.com/en-us/microsoftteams/troubleshoot/teams-administration/clear-teams-cache and reinstall Teams but the problem remains.
Looks like this problem occurs on computers with Windows 10 installed years ago. With newer installation there has not been any problems.