Normally this should not happen. It should not clear data outside the fix statement.
Please have a look if you can find the statement in the Application Log. This might give more information what happened.
When an invalid member selection is made in a FIX statement (i.e. @DESCENDANTS("Forecast") where Forecast has no children) then the FIX statement will be ignored and all Members will be selected.
You could rather use ClearData as it is just one account and account is dense dimension. A normal best practice though
There would be any performance implication though. Still it is really strange that it has cleared off all the accounts