I had a similar issue except that windows would, rather than not remember the key, it remembered the wrong key.
The problem was with a win7 Laptop connecting to an iPhone ios7 "Personal Hotspot" .
The Password/Key that I saved worked the first time it was set up, but subsequent connections would not accept the key.
When checking the key windows had registered for that SSID , it was some odd string of characters, but always the same.
The problem turned out to be that when Windows encrypted, and/or decrypted the Wireless Connection configuration, mentioned previously as being saved under C:\ProgramData\Microsoft\Wlansvc\ it could not cope with "non-standard" characters in the SSID.
The name of the iPhone, thus the SSID, was set to "Brian's iPhone". (see the apostrophe)
I could connect with this SSID with our desired key, but subsequent attempts did not recognize the key.
Changing the iPhone's SSID to "Brians-iPhone" solved the problem.
Since there is no documentation from Microsoft stating that an SSID must contain only certain characters, and since no warning pops up at any time... Im calling this a BUG.
My gut feeling is that the programmer that created the encryption/decryption script for this function needs re-visit the script and fix the problem.