Reserved tag 'NOT_REVIEWED?

Whenever I start the robot including ‘–exclude NOT_REVIEWED’ , I get the following error:

[ ERROR ] Suite ‘XXX’ contains no tests not matching tag ‘NOT REVIEWED’.

But suite ‘XXX’ does not contain any tags at all and the same happens with all test suites - no matter, what tags they contain.

Anything else, that I tried - including ‘–exclude not_reviewed’ works just fine.

Is there anything special about NOT_REVIEWED with capitals?

Yes, NOT is reserved word, more details in Robot Framework User Guide

I know that part. But the reality is, that it doesn’t work. WHY?

To be more exact:
I know, there is no documentation, that it would be reserved,
and according to the documentation it should not be reserved.
Nevertheless, it behaves as if it is reserved.

Did you see this one:
In addition to specifying a tag to match fully, it is possible to use tag patterns where * and ? are wildcards and AND, OR, and NOT operators can be used for combining individual tags or patterns together:

It doesn’t take NOT_XXX as a whole neutral name, but it takes it as NOT XXX.
It would perform only test suites with the tag XXX - as the tests not tagged with XXX are excluded.