Selecting a skin from a custom skin pack resets every time I reopen the game

Steps to reproduce:

1. Have a custom skin pack
2. Select a skin from it
3. Quit the game and enter again

Expected results:
The skin is kept when reopening the game.

Observed resulst:
The skin resets to either Steve or Alex.

Note that this is only with custom skin packs, marketplace packs and custom skins still work properly.