Historically you have never had to specify repetition 1 of a field, i.e. if you wanted to use Set Field or any other function on repetiion 1, you just specified the field name. Subsequent repetitions always needed to be specified, e.g. MyField. I have found a bug both in Set Field and also in Hide Object When, where you MUST specify the first repetition or it will not work.
Please see the attached video for a demonstration of the issue.