You can try running the below script in your AD connect server and then check if the passwords get's synced.
$adConnector = "on-premises AD connector"
$aadConnector = "Azure AD connector - AAD"
Import-Module adsync
$c = Get-ADSyncConnector -Name $adConnector
$p = New-Object Microsoft.IdentityManagement.PowerShell.ObjectModel.ConfigurationParameter "Microsoft.Synchronize.ForceFullPasswordSync", String, ConnectorGlobal, $null, $null, $null
$p.Value = 1
$c.GlobalParameters.Remove($p.Name)
$c.GlobalParameters.Add($p)
$c = Add-ADSyncConnector -Connector $c
Set-ADSyncAADPasswordSyncConfiguration -SourceConnector $adConnector -TargetConnector $aadConnector -Enable $false
Set-ADSyncAADPasswordSyncConfiguration -SourceConnector $adConnector -TargetConnector $aadConnector -Enable $true
NOTE: Change the "on-premises AD connector" and "Azure AD connector - AAD" values according to your AD connect connectors
Let me know if you have any further questions on this.
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