There is an issue with the Welcome message but it is normally in part a misunderstanding on how it works.

Default does not mean that we enforce that value to be used. It means that the value will be used when creating a new Room. But the teacher has the last word.
  * If no global default is defined through  env variables or plugin settings, the default string from translations is used when creating a new Room

• If a global default is defined either through ENV variable or plugin settings, that string is used when creating a new room
• Either way the Teacher can edit the welcome message. Remove it if necessary Once the room is created

When the Welcome message is removed from the Room, the one used should be:

1. The one defined in the plugin settings, if it is not blank
2. Or, the default string from translations as a fallback.

Now here is the issue/bug. No matter what, when the Welcome message is removed from the Room, the one defined by the default string from translations is used

Workaround at this point. # Set up a Welcome message per room (which is a hassle though)

1. Edit the translated String in Moodle and set it up as the admin wishes