You may be right, if they execute the same or similar logic in the background, probably they have considered routine 104 as obsolete.
Unfortunately if your company does not keep up-to-date information on the customer-specific developments in a repository - asking a developer for assistance could be the the fastest and most efficient way to find the root cause.
In case the problems appeared recently and it is not right after an upgrade project or a major development import, it might be good to check whether somebody implemented a sap note that could result in unexpected behaviour of the custom logic. In an ideal world a sap note implementation would not have an impact on user exit logic, but it happens.