Collibra DQ unable to join dataset using @

For a specific dataset created, Collibra is not able to see it as a dataset name and is throwing error - Exception with freeform sql rule check: extraneous input '@' expecting ...

Usually if @dataset is registered, the query will change the name of the dataset from @dataset to <dataset>_dataset in the Rules preview.

The following conditions were met - 

• name of first dataset matches name of main dataset which rule is created
• joining dataset has a quality >75%

May I know what other configuration has to be done?

Note once I change it from Freeform SQL to Native, it works. Something is wrong with the freeform SQL? However, I need to store the rule breaks in metastore hene native SQL is not an option even though rule is working fine there.