I tried the new version 1.7.0.105 - 1325 a problem.
Also a change in Preferences.lua isn't solving the problem.
My Steam is installed at : D:\Games\Steam.
I get also an auto corretur back to "C:\Program Files (x86)\Steam\" every time.
Can you send me the log file TFGMLog.txt in a PM. It is located in the same folder as TFGM and teh log feature is enabled by default (if you haven't turned it off).