Adjustments to the ui scale are not saved properly and reset if you do a /reload.
uiScale and useUiScale are absent from the WTF/config.wtf after you /reload or exit the game.
They will however be saved in the config.wtf if you only exit to character screen. If I check the config.wtf at that point the correct values are in the config.wtf. But those values are removed on exit/reload/login.
p.s. I repaired the PTR installation twice. I ran no addons. I reseted all settings to default multiple times. Bug is consistent. Other interface options will be saved properly. (like window size, combat text settings etc.)
http://eu.battle.net/forums/en/wow/topic/17613721386