Create a custom subflow and use it in the Alert Management Rule. Here the customer is trying to update the alert record fields.
All releases where Alert Management Rules apply.
Input defined to read values from fields which doen't have "ah_" prefix in the name.
Make sure to follow Alert management Template when creating your own subflows. If the input is not given with necessary prefixes then you will see the above error mentioned.
Of course this is one of the reasons. Also, check the Business Rule Update Variables Model Information is OOB and up to date.