The @r tag does not work when you if tag an entity. (example: @r[type=pig])
It will however work if without the type tag. (example: @r)

Steps to reproduce:
1) Spawn 3-4 pigs and try the following the command: execute as @r[type=pig] run say hey
You should notice nothing happens.
2) Now get a friend on your Minecraft server and execute the following command: execute as @r run say hey
It works flawlessly if you do not tag any entities.