Yes, you are right. Normally it works like that.
Meanwhile I found out, that the problem was a popover which was connected with the constrain.
I used a textfield within in a popover for the constrain. It was triggered by "exit popover".
When Enter was pressed , the popover was closed after the quick find, but triggering another script, which was quit accidentally using the wrong "quit script command".
The result was, that the script command "constrain found set" was not executed.
Hope my explanation is not too confusing.