Changing a password on an account does not update the last successful password change time stamp (4340367)

This is by design where an account that is a member of a Password Sync Group will follow the Change Password profile schedule that is applied on that password sync group. Therefore, a forced change password task that is ran at the account level will not trigger an actual password change on the the target account but rather it will set the account to the same password which is currently stored in SPP as the assigned password for the Password Sync Group to keep it in sync.

Password Sync groups will change the password in bulk on all accounts that are members of the sync group based on either the change password schedule that is assigned to the sync group or can also be force changed on demand for all the accounts in that sync group from Settings > Password Sync Groups > Selecting the group > Click on Change Sync Group Password  icon. > Click Yes to acknowledge the prompt.