There could be two reasons why your setting is not permanent:
1.application that tries to set setting don’t have rights (in this case explorer.exe)
2.some other software change the value back after restart (could be malware)
So try exit explorer.exe and then run it as administrator (to check first possibility) or try to disable all startup apps and also all non-Microsoft services (both could be found in "msconfig"), restart and try to disable Touch Keyboard one more time.
I none of this works, you can use system restore to get back to time, when the keyboard wasn't checked (if you got restore point old enough).