Empty Friends list after I went to another app and then back to Minecraft

Steps to reproduce

1. Open a world on a PC
2. On the same network, open Minecraft on iOS.
3. On the iOS device, navigate to the Friends tab. You should see the PC user/world listed there.
4. On the iOS device, switch from Minecraft to another app, or just press the home button. Then switch back to Minecraft.
5. Repeat these steps with the devices swapped, i.e. with the iOS device hosting the world and the PC switching to another app and back (PC must be in fullscreen mode).

Expected result

The PC user/world shows on the Friends list in step 4, or vice versa for step 5.

Actual result

The Friends list is empty.

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LAN game seems not work for me. My friend and me are at the same WiFi. He hosted a multiplayer game and I can not see any worlds showing in my friend list after I went to another app in between and then back to Minecraft.