Service program reference problem

We have a problem with the relation between a program PGMA and a service
program SRVPGMB. As far as we know, this has never been a problem before.
I will try to explain the situation.

PGMA was compiled in a test environment. In that environment, service
program SRVPGMB has 6 procedures. Afterwards, the program was transferred
to the production environment. In that environment, SRVPGMB still has 5
procedures. (The sixth procedure was added in a project that could not be
implemented yet. The sixth procedure was added to the end of the service
program.) Now when program PGMA is called, this results in error MCH4437
Program import not found. We thought that the fact that the sixth
procedure is missing in the version of the service program in the
production environment, would not do any harm (provided of course that the
program does not actually use the new procedure).

We have recently upgraded to V5R4M0 (yes, we are a bit late in doing so).
We suspect that this behaviour could somehow be a result of the upgrade.
So far, we have not found any clue. We hope that someone recognizes the
problem described above, and can give an explanation (and a solution) for
it.

Note that the error is NOT caused by a signature violation. The signatures
of SRVPGMB are the same in the test environment and the production
environment.