The startscript function targeting inherits object ID instead of instance ID
If an NPC has a script on them that starts another script, using startscript with no targeting should cause the script to be targeted to the instance ID of the instance running it, not the object ID. If it's targeted to the object ID then it will be fine if there is only one instance since the instance ID of the first one is the same as the object ID, but if there are multiple then it will always target the script to the first one. Test by putting 3 guards (same object) in the mournhold cell with a script that starts another script to add 1 to their attack bonus. Two of the guards will have an attack bonus of 0 and one will have 3.
(RM-4417 from redmine: created on 2018-05-17 by halbe null, , closed on 2018-05-17 by nobody, duplicates #2311 (closed))