Control-s saves the document even when there is nothing new to save. I think it disabled the save when there was nothing to do. Was this changed or is a regression bug in Calc?
I think it was an intentional change. There was previously an option to always allow saving even if no changes had been made, but the default was, as you recall, that saving was only enabled if changes had been made. From release notes, it looks like that option was removed in LibreOffice 5.1 (at least, I think this is referring to the same option):
Removed configuration options * Removed /org.openoffice.Office.Common/Misc/AlwaysAllowSave. Saving is now always allowed. (Maxim Monastirsky)