To my knowledge, there was no change in how sliding working in FileMaker 13 as compared to earlier versions.
Then what in the world is going on?
Do yo think my file got corrupted? I've never seen sub summaries behave this way
I don't know enough detail about what you have set up to tell.
A corrupted file or layout is always a possibility when you see such a change in how your file behaves, and it's rare but not unheard of for a file to have hidden damage that does not appear to affect the behavior of the file until you either change Database versions or Change OS platforms.