What version of ArcGIS Pro are you using? If it's not working like it says in the documentation (Work with a legend—Layouts | ArcGIS Desktop), I would tend to think it's a bug, unless it occurs only certain specific times.
You may also want to refer to this similar, previous thread: https://community.esri.com/thread/213422-arcpro-legend-will-not-resize-symbols-or-labels
I've tried to reproduce this with the latest release, 2.4.3, but haven't been able to. All my changes apply and are saved. Could you reach out to tech support and see if they can reproduce it with you?