What git hash are you using? I once had an issue about this. The data modification is being dispelled when the mobs element changed.
but it was fixed. Check this LINK
If you are using a git hash higher than the fix. Try to execute this code and check if the mob data is being changed.
.@mon_count = 3;
monster "payon",0,0,"Bigfoot (Weakened)",1060,.@mon_count,"Room of Dungeon::OnMyMobDead"; //Bigfoot
for (.@i = 0; .@i < .@mon_count; .@i++) {
.@mobgid = $@mobid[.@i];
setunitdata .@mobgid,UMOB_LEVEL,5;
setunitdata .@mobgid,UMOB_MAXHP,100;
setunitdata .@mobgid,UMOB_HP,100;
setunitdata .@mobgid,UMOB_ATKMIN,40;
setunitdata .@mobgid,UMOB_ATKMAX,45;
setunitdata .@mobgid,UMOB_DEF,15;
setunitdata .@mobgid,UMOB_MDEF,15;
setunitdata .@mobgid,UMOB_FLEE,70;
// Will check the monster current data if modified by the above script
getunitdata .@mobgid,.@param;
dispbottom ""+.@param[UMOB_LEVEL]+"";
dispbottom ""+.@param[UMOB_MAXHP]+"";
dispbottom ""+.@param[UMOB_HP]+"";
dispbottom ""+.@param[UMOB_ATKMIN]+"";
dispbottom ""+.@param[UMOB_ATKMAX]+"";
dispbottom ""+.@param[UMOB_DEF]+"";
dispbottom ""+.@param[UMOB_MDEF]+"";
dispbottom ""+.@param[UMOB_FLEE]+"";
}