Have you ever created a Hide/When formula on a line in a form and when you tested it, the results are not what you expected?
In my particular case, I have a form that requires multiple fields with Hide/When formulas. The fields have the same formula, but are field specific. From Designer, I cut and paste the code into the appropriate area, alter the formula for the field, then save the form and test my results in the Notes Client. What I found was the Hide/When reverted to the original formula from the paste.
The solution is simple. Close the form in the Designer before testing your formula in the Notes Client. The Hide/When formula will be pushed back from the UI Document in the Notes Client to the still open form in the Designer.