Thanks for your feedback.
I didn't use Partner role LS during my tests (only partner type LS and MN25 was done correctly), but have since moved on to find another solution (creating new message types using same basic types).
I’ve also created a log with SAP OSS. Herewith their feedback:
Too many recipients found for message type SHPMNT in the ALE model
Message no. B1130
The message you are receiving explicitly states that this message
type cannot be sent to multiple recipients.
Please change the ALE distribution customer model so that it allows only one recipient system for the message from this system.
Please note, that not all messages have this restriction. This
restriction is specified by the business scenario.
In case of master data you can send the same data to
different receivers. It's the case if each receiver has to hold the
same master data.
From the process it's as following: An application
creates a master idoc and will call the ALE layer. In that layer we
will check the distribution model (tx BD64)
and we'll send the data as idoc to each receicer which was found.
(from ABAP technique fm MASTER_IDOC_DISTRIBUTE or ALE_IDOCS_CREATE is
called).
For all scenarios which use message control we have just 2 ways to
create idocs. There are the mediums EDI (6) or ALE (A). The
result is the same you get just one idoc. The way to find the
receiver is different.
Using medium EDI the receiver is the partner
which was found already via message control and which was saved
within the partner profile (TX WE20). The distribution model (tx BD64)
is not used in that case.
Using medium ALE it doesn't matter which receiver was determined
via message control. We look via the distributen model for a unique
receiver for the message type in question.
Either we've just one pair of sending and receiving system or the
pairs have different filters (LIFNR as example).
From the technique the application (tx VA01 as example ) creates
a message record (entry table NAST) with the information about the
used medium. If you use the collect mode report RSNAST00 will call
the reports behind the mediums. These are edi_processing or
ale_processing you can find within program NASTEDF0.
If you wish to create more than one idocs despite there is just one
message record I can't suggest a standard way.
One way is to create just one idoc it doesn't matter using medium
EDI or ALE. This idocs can be posted to a port ABAP/PSS.
check tx WE21. In such port you can double such idocs to further
receicers.
The other option is to use own reports similar edi_processing or
ale_processing which you can use to process message records.
In table TNAPR you find all called reports for each application.
To maintain you own reports you've to use tx NACE, I think.
In these reports you could call fm MASTER_IDOC_DISTRIBUTE as example
and you could create more than one idoc per call.
I’m confident this thread will assist others with similar issue.
Regards