The name of the dialog file was the key. I was able to search on that and I found the problem! The authors of MotSC are using the StartDialog() command. For example, see the actions in the following script block from AVAL01.BCS:
IF
Global("Z!Aval_TsukiName","GLOBAL",1)
See([ENEMY])
InParty(Myself)
!StateCheck(Myself,CD_STATE_NOTVALID)
See("Kagain") // Kagain
InParty("Kagain") // Kagain
Range("Kagain",10) // Kagain
!StateCheck("Kagain",CD_STATE_NOTVALID) // Kagain
THEN
RESPONSE #100
SetGlobal("Z!Aval_TsukiName","GLOBAL",2)
SetGlobalTimer("PartyBanterPause","GLOBAL",600)
ActionOverride("Kagain",StartDialog("Z!16AV_I","Aval01")) // Tsuki
END
The problem is that StartDialog() changes the actor's dialog file (
https://gibberlings3.github.io/iesdp/scripting/actions/bg1actions.htm). So, this script block will permanently set Kagain's dialog file to Z!16AV_I.DLG.
137 StartDialog(S:DialogFile*,O:Target*)
This action instructs the active creature to start the specified dialog with the specified target. The dialog can be initiated from a distance and must have at least one state with all its top level conditions true else it will not initiate. The active creature has its dialog file permanently set to the file specified by the DialogFile parameter.
I assume the author of MotSC thinks StartDialog() functions like StartDialogOverride(), which does not change the creature's dialog file. Unfortunately, StartDialogOverride() is not available for BG1, so you can't just change the StartDialog() commands to StartDialogOverride() and have it work in BG1 (that would work in Tutu/BGT/BGEE). To maintain BG1 compatibility, the author of MotSC will need to rewrite these dialogs.
At this point, you need to bring this to the attention of the MotSC author(s). They need to fix the StartDialog() problem.