Hi @Jasper Samuel
There are likely several reasons for this difficulty in resetting your password.
My best guess is you were experiencing a problem with your on-prem Active Directory account not synchronizing with Azure AD.
So it makes sense that you had to login with your old account to change your password.
This would have updated your on-prem Active Directory server with the new password, which would have pushed the update to Azure AD.
I wonder if this will happen the next time you reset your password, if so then your Azure Administrator may need to look into whether passwords are being synchronized properly between Azure AD and Active Directory.
More information on these sorts of problems can be found here:
troubleshoot-pwd-sync
If this helps, please accept the answer and upvote.
Either way, have a super day.