Fully cooked, but requires a minimum internal temperature?
There are arguments of the universal validity of such rules, but there has long been an argument that reheated meat that was pre-cooked should always be brought to full safe temperature.

The general idea is that cooked or not there is bacteria present.  If heated to only the 100-120F range, you have reached the temperature of highest active growth of many pathogens, without killing them.  Some will argue that this is safe, as long as you eat the item immediately, not let it sit and give time for that bacterial growth to bloom.  I would even put credence to this for items which I knew the preparation and handling history, but for prefab type foods, like ground meats and especially poultry, I personally would be reluctant to take risks, I would bring it to temp.  

There also has always been disclaimers that the reheating to temp rule only applies to if you reheat at all, for instance, medium rare roast beef is often eaten cold as leftover, but the rule stated if you reheat, the reheat to what would have originally be considered a safe temperature, not part way.  With something like a prepped burger, again, I personally would not eat it cold, and just thawing it might put you into an unknown zone.  I would heat it.