What do you mean by ‘side effect from another process’?
Usually you would not put validation for user feedback on events. Can you explain, why it’s not option to do that in a microflow?
On the other hand – are you really using Mendix 7.13? The response you get could also be some Bug.
regards, Fabian
If you want to validate the entity and show a message to the user it is best to do this with a microflow button, for example a custom save button. You can check the entity there and show for example a validation feedback for the user to see what is wrong.