Welcome to the Asana Community Forum @anon54472806!
If you want the task to be moved instead of multi-homed make sure you select this option when creating the rule:
Then it works like you explained:
To do this you can set up a rule that is triggered by any tasks added to a special section or for tasks that have a special selection for a custom field drop down for example.
To avoid this from happening the trigger for the rule set up in project A might need 2 conditions.
Now in your case you mentioned any task added via the form to project A should be moved to project B. This basically means for the rule trigger if you set it up to all tasks added to this project from form submissions only then it would not be triggered for other tasks.
Can you try this out?