App is installed for 2nd Android user. User account is restricted via Google family link, but that does not appear to have any influence.
Saving additional servers is working fine if MC user is 1st user on device.

Additional servers are only shown while the MC app is running.
After stopping and starting MC again the list of additional servers is empty, regardless of what was entered during previous runs of the MC app.

Manually copying a file external_servers.txt from a working setup to /mnt/user/10/primary/games/com.mojang/minecraftpe doesn't change anything.

But /mnt/user/10/primary/games/com.mojang is definitely used for the 2nd user:

• With ADB other configuration files can be seen at that location:

  adev:/ $ ls -la /mnt/user/10/primary/games/com.mojang/minecraftpe
  total 30
  drwxrwx--x  2 root sdcard_rw  3488 2021-01-03 21:07 .
  drwxrwx--x 13 root sdcard_rw  3488 2021-01-03 21:08 ..
  -rw-rw----  1 root sdcard_rw    20 2021-01-03 21:07 clientId.txt
  -rw-rw----  1 root sdcard_rw  9727 2021-01-03 21:40 options.txt
  -rw-rw----  1 root sdcard_rw 10796 2021-01-03 21:39 valid_known_packs.json
  adev:/ $ 
      

• Also /mnt/user/10/primary/games/com.mojang/minecraftWorlds is used to store worlds for the user in question.

Reproduce

• create 2nd user on Android device
• install MC app for 2nd user
• start app
• go to play -> server tab
• enter one or more additional servers
• push Save for each entry
• stop app
• start app again

Actual result

• list of additional servers is empty

Expected result

• all additional servers entered should still be shown