Hi @Reck, Max
At this time, MS Teams using the Display Name in Azure Active Directory to assign this. If you are syncing users to your tenant via Azure AD Sync or some other tool, you may need to change the ordering in how you integrate between the systems.
If you aren't syncing the users you can either change it directly in Azure Active Directory or use PowerShell Get-AzureADUser and Set-AzureADUser commands to achieve changing the Display Name.
Setting it in Azure AD
Go to https://aad.portal.azure.com/#view/Microsoft_AAD_IAM/UsersManagementMenuBlade/~/MsGraphUsers > Choose the User > Edit Properties > Edit Display Name
Get-AzureADUser
https://learn.microsoft.com/en-us/powershell/module/azuread/get-azureaduser?view=azureadps-2.0
Set-AzureADUser
https://learn.microsoft.com/en-us/powershell/module/azuread/set-azureaduser?view=azureadps-2.0
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