Fix Version is a multi-value field. Thus the Except() function will only filter out issues that have only the particular member (or members) specified.
Depending on the report structure, there are multiple workarounds. If you use the Issue dimension in rows, you can add the “Issue fix versions” property to the report and filter the report rows. See an example below:
Without the Issue dimension “Issue” level in the report, a simple subtraction could suffice in some cases. See an example for a calculated measure below: