texture pack subpacks not working correctly

This bug causes significant lag with subpacks on, making them basically useless.

to replicate:
1. turn on error logging in settings>profile
2. download the test pack or another pack with subpacks
3. activate the pack
4. restart the game
5. click any button that takes you to a different screen

observed result:
the next loading screen will take 5-10 sec (for me, my device is somewhat low end) to load and will show a loading bar
it will not appear again until you restart the game or exit and leave a world/server
this error will also be thrown:
19:31:39[Texture][warning]–Failed to find either texture set or image file of name (ms–appx:/data/images)

expected result:
an unaffected loading screen with subpacks on