First, you have to find out what the collation of the table is. Therefore, you need to run this query:
Find the collation of the tables starting with _.
In our case this is: utf8mb4_unicode_ci
Also, find your Write Back scenario table and have a look at the collation.
In our case this is: utf8mb4_0900_ai_ci
These collations are incompatible, so lets change it!
You have changed the collation of your write back scenario tables.
The last step is to change the default collation of the database. Therefore, this will not happen again in the future.
You should be able to join tables again in Tableau.
Have fun with Write Back Extreme!
Learn more about Write Back Extreme: Product Guide