After testing a custom resource pack using the development_resource_packs folder, when you later deactivate it and delete it from that folder, an entry for it appears in the world's Resource Packs > Realms Plus Packs section, and its description says "This pack is missing!" This is an error because it is not a Realms Plus pack, and because getting rid of it in-game is difficult and unintuitive.

Steps to reproduce:

1. Create or obtain a trivial resource pack in source form. (You can download the attached "Map Helper - Yellow.mcpack" file I created as a workaround for MCPE-27720.)
2. Rename the file extension from ".mcpack" to ".zip" (if it isn't already in .zip format).
3. Extract All to the folder "%LocalAppData%\Packages\Microsoft.MinecraftUWP_8wekyb3d8bbwe\LocalState\games\com.mojang\development_resource_packs".
4. In Minecraft, create a simple test world. If the world loads, save and exit.
5. In the test world's settings for Resource Packs>My Packs you should see the resource pack you just installed for testing. Activate the pack.
6. Select the pack in Main Menu>Settings>Storage>Resource Packs, then delete it.
7. Look at the test world's settings for Resource Packs>Active. Note the error message "This pack is missing!". Select the entry and deactivate it.
8. Look at the test world's settings for Resource Packs>Realms Plus Packs.

Observed results:
You see an entry for the test pack with the error message "This pack is missing!" There is no corresponding entry (erroneous or otherwise) in the settings for Resource Packs>My Packs.

Expected results:
There is no entry for a missing pack in either My Packs or Realms Plus Packs because it was only installed temporarily in the development_resource_packs folder, which is meant for testing.

Screenshot showing the results: