since the Mail app on iOS will always, without fail, need to ask for a new password that has been changed, the likeliest possibility is that in fact the password change you thought made was not actually successful and that the old password is still actually the password (or, even that your old password is in fact the same as the new password).
if you think that the above is not correct, then of course what you should do is to again change your password on the mail account (if possible, on a different device) and then try yr iOS Mail on yr iPad and see if it requires entry of a new password. if it does require a new password, great. but then change it yet again, this time from yr iPad. and then see if Mail requires the new password.