REST endpoint does not return Last-modified header for GET operation if the entry was stored via HotRod or Embedded cache

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).