Client downloads the world from their server, but their player data in Singleplayer is reset to the time when they first uploaded the world onto server.

This article explains how to solve the issue where singleplayer world's player-data was lost during world upload progress.

That has happened because their level.dat file includes the singleplayer player data. Open their level.dat file with NBTExplorer and delete the "Player" section. Then save, and reupload to the server.

Make sure to delete the level.dat_old file before starting up the server!