When an entry is stored via HotRod an embedded cache, and later retrieved via REST,the Last-modified header looks like this:

Last Modified: null

Another observation is that when the entry is stored via REST and retrieved via REST again, the Last-modified header is:

Last-Modified: Thu, 01 Jan 1970 00:00:00 GMT

This is obviously wrong and this was working correctly with JDG-server which contained ISPN 5.2. OTOH, when I set "timeToLiveSeconds" header on the request, the Last-modified is properly set (not to 1970).