Global Resources, such as your resource packs, are reset upon reopening the game. This can be very infuriating for someone that has a lot of resource packs and has to scroll through their list to find which one they used upon exit.
An easy fix is to keep Global Resources no matter the circumstances.
Thank you for your report!
We're tracking this issue in MCPE-39045, so this ticket is being resolved and linked as a duplicate.
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