Same problem here. As far as I can tell, it happened when signing out of iCloud on PC, and the warning that the files on the PC will be deleted. Thinking these are merged files, that did not seem so significant, but it is necessary to perform some root scans with anti-virus software. However upon syncing again with iPhone, those contacts were indeed lost. Admittedly I may have had Outlook contacts open when syncing, as there was also some data corruption to a contact file. It is not clear what happens should Outlook be in use or not.
In the future I really need to think about what sequence to follow when plugging in iPhone and allowing automatic sync, as instruction is not clear nor are there warnings. And one might expect this for cross-platform utilities. I have changed the default of "sync automatically" in iTunes to not. I will also make sure Outlook is fully synced with iCloud first, then the program closed, before manually syncing with an iPhone.
The solution of back-up is good, but pinpointing the correct one to use is difficult when not knowing the exact date the corruption took place.