You are experiencing an issue with concatenated SMS. The MAP session closes despite a flag that notes that "more messages are waiting (in the MO-Forward)”.
Further, you are also getting failure for the “More Message to Send=0” while concatenated SMS are success for value “More Message to Send=1”(No more message are waiting for the MS in this SC).
When the RTR sends TCAP-end to the first segment of a concatenated message with MMTS flag, the other segments are not delivered.
Unfortunately there is no workaround available on the SMSC side. You can request the originating SMSC to send concatenated messages without MMTS flag set using an individual TCAP dialogue for each segment.
This is a known bug that was addressed in R04.12.06.01 - NN28975 - More Messages to Send does not work as expected (MT-SIPT). You can plan an SMSC upgrade to the latest GA release to address this bug.