I checked some other cases that had a similiar question and it looks like you need to add the value "ApplicationIntent=READONLY' as a customization name when using MultiSubNetFailover. I think the article below may help.
I hope that helps.
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Did you find the answer for this issue?
We are running into the same problem to configure MultiSubnetFailover. The link in the article you put is broken. Could you post the updated link?
In addition, is it possible that Tableau can add the options in the interface to set this option so that users don't need to go to file to edit it manually.