Hi,
the Arduino Windows App installs the hardware platform files in program files/WindowsApps. You are not allowed to modify platform.txt there.
You cannot redirect the platform path to C:\Users<user>\AppData\Local\arduino15\packages\arduino\hardware\avr\1.8.2 from where the version of windows installer installes the files.
So, using the Windows App is a great disadvantage!
King Regards
Ulli