Thank you for your post.
I am unable to replicate the issue with FileMaker Pro 14.0.4 under Mac OS X 10.10.5 (and Mac OS X 10.11.1). That is, I can enter a calculation in the Specify Calculation window of "Hide object when", and when I click OK, the calculation appears in the "Hide object when" field of the Inspector.
Is it possible you have a leading carriage return? This would then show a blank line in the Inspector?
I narrowed the issue. I have a Floating Window in browse and a normal window in layout.
In the normal window, when I make the "Hide Object when" in Specify Calculation, then click OK, the Floating Window seems to interfere (the window flashes) and the calculation from the normal window is not saved. No interference when I enter the Hide calculation in the Inspector.
Thank you for the additional information.
Our Development and Testing departments are already aware of this issue as it was originally reported by another Community member. See: Hide condition changes do not commit when saving layout in 'background' window
I have attached your post to the original report. When more information becomes available, I will post again.