Try to force a full DirSync via AD powershell command Start-ADSyncSyncCycle -PolicyType Initial and then see if it make any difference, thanks.
Moroever, if the issue persists, in this sceanrio, please connect to Microsoft 365 Azure AD powershell and then run the following commands then share the results to me, for how to connect to Azure AD powershell, please refer to Connect to Office 365 PowerShell > Connect with the Microsoft Azure Active Directory Module for Windows PowerShell ( not your AD powershell)
Please run the commands one by one:
- (Get-MSOLCompanyInformation).DirectorySynchronizationEnabled
2. (Get-MSOLCompanyInformation).DirectorySynchronizationStatus
On another hand, I'd like to collect some other information and help you further troubleshoot it
- If you make any changes to your AAD connect sync service like changing any settings?
- Please provide a full screenshot about the whole error when you tried to sync your new users.
Let me know if you have any further questions.
Please "Accept the answer" if the information helped you. This will help us and others in the community as well.