If you check the import log you will be able to find which value needs to be updated.
If you can send the import log, we can investigate and probably give you a workaround
Thank you very much for the reply 
However, this question has already been answered in IFS Ref 445316, where it was confirmed as a bug. This bug is expected to be addressed in the 1.10 release.
To help anyone else who might face the same issue, I’ll provide the context and a workaround:
This issue was encountered in IPaaS 1.8, and the reason for the issue seems to be that onBehalfOfUser is not an actual attribute on any Assyst entity, but is evaluated as one during import update evaluation. As a result, ETM import evaluation gets a NULL value from onBehalfOfUser, and will always sets the record to be updated as the outbound.onBehalfOfUser has a different value than NULL.
Example from karaf.log:
2025-03-26T08:24:03,125 :CET] | INFO | Camel (importToolProcessingContext) thread #28 - ImportRecordSplit_MEDIUM | DatamapperProcessor | 188 - com.axiossystems.integration.importtool - 1.8.0.STABLE2024-04-12T114824Z| Change in value of field onBehalfOfUser (from null to ZZ_CONTACT_USER_GATEWAY). Record marked as requiring update.
The workaround is to set a value in the outbound.self.onBehalfOfUser to so that the import update check “finds a value” from Assyst that is the same as in the imports outbound.onBehalfOfUser
The information as conveyed in followup to the support request:
//Fudge the self (AKA Record to Update) data and create a value for onBehalfOfUser
//to avoid redundant dirty checks
outbound.self.onBehalfOfUser = 'SOME_USER';
//use that value to set the onBehalfOfUser - Don't Repeat Yourself
outbound.self.onBehalfOfUser;
PLEASE NOTE - we can't guarantee that this workaround will work in any future, unreleased version.