If you open Minecraft after a while of not playing, it won't "load the resource packs" and your custom skins will no longer be saved. How to recreate:

1. Open Minecraft
2. Save at least one skin to the quick skin select screen.
3. Close Minecraft
4. Wait at least a day. Opening Minecraft again immediately won't activate the glitch.
5. After a day or longer, open Minecraft again.
6. The glitch is likely active now if the "loading resource packs" screen does not appear. Still, now check your skins. If they are empty, default Steve, or default Alex, you have recreated the bug correctly and now you have to start your skin all over again.

Okay, so I tested again after figuring out my proof of the bug. Turns out you only have to wait for about 15 to 20 minutes. My current skin was SUPPOSED to be the Mask guy from the ZOMBIES pack. Sorry about the poor recording method and quickness. I had a limit on how long the video could be.