Random string is displayed in the right bottom of audience screen. What's this?

This is called as "Watermark" by user ID format.

  ** This isn't any error or any bugs, expected behavior based on distributor's intended setting.

This is visible on the audience's players only if all following settings are enabled on Security tab,

1. "Token" is chosen on Visibility

2. "User ID" type is chosen after watermark is enabled

Please refer to the following article for detail,

• Set up Security Features - Setting up watermark