I just wrestled with that one this weekend. Here's what is going on:
When you use button setup, to turn a layout object (such as your field) into a button, FileMaker creates an invisible "button object" and groups it with that layout object. Just as when you select several objects and group them, object names and this "hide when" setting seem to disappear as they only apply to one of the grouped objects and you can only see them if you ungroup the group of objects so that you can select each object from the group individually.
Of course, if you ungroup a button, the invisible button object is deleted so it's best to set up the object as a button first and then specify object names and "Hide when settings" so that they are visible and accessible.
Ah, that makes sense. Albeit, it is not that evident as your general user and can lead to some confusion as well as the loss of some nice calcs you have to re-write.
Maybe there could be a warning like the one which says something similar to the 'ungrouping objects' warning you have a script applied e.g.
The selected object includes a 'Hide Object When' calculation. Removing this script will also remove the 'Hide object when' definition. Do you want to proceed?
Maybe that's overkill, but it would certainly help to either modify the behaviour or to warn you of the unintended consequences of your actions.
You are not the first to suggest that such be made more obvious--just the first to point out that "hide when" is another such case (Object Name being the other) where this can be an issue.
You could make a feature request if you want: http://www.filemaker.com/company/contact/feature_request.html
Thanks Phil, always appreciate your great input on the forum. I have added the feature request.