When looking at the player .dat files before (when server player had their in game progress) and after they log out then back in (all progress gone) I notice that two new dat files are created for the server player : server_(string of numbers).dat and (string of numbers).dat. The old server file is still there showing all in game progess contained in it. The old (string of numbers).dat file no longer links to the old server file but to the new sever file, as well it has a tag that links to the new (string of numbers).dat file. In short all reference to the old player data with progress is lost but the file is still there
UPDATE
Appears to happen when a server player logs out of their account then back in then joins the game.
When looking at the player .dat files before (when server player had their in game progress) and after they log out then back in (all progress gone) I notice that two new dat files are created for the server player : server_(string of numbers).dat and (string of numbers).dat. The old server file is still there showing all in game progess contained in it. The old (string of numbers).dat file no longer links to the old server file but to the new sever file, as well it has a tag that links to the new (string of numbers).dat file. In short all reference to the old player data with progress is lost but the file is still there
UPDATE
Appears to happen when a server player logs out of their account then back in then joins the game.