When using a tag to set an alarm limit in an analog tag there is no validation that the limit tag continues to exist. When you first set up the alarm limit, the alarm limit tag is selected from a list of available tags. OK. But if you delete or rename the limit tag later there is no warning nor overlay to indicate that the limit tag is invalid.
I will add this as a feature request and see if we can get some sort of visible notification in there to let you know that you have selected a tag that does not exist any longer.