AFAIK, this can happen when the value of the date or date/time key in Source mode wasn’t written following the ISO8601 format (as it’s the only format allowed for date and date/time types of keys in Properties) and Properties can’t parse the value of the key correctly 
…
I could still be wrong though 
… But reading your post made me think about this similar issue here: {{date}} not returning today's date in templates - #5 by Pch