Weird indexation problem

I have a database with classical music works. It has three relevant fields: composer, cataloga and shortname. Cataloga contains opus numbers - left padded with spaces for easy sorting. When there is no opus number the field is empty. It is the order of the empty fields that puzzles me. It is like there is some hidden sorting happening.

When I do "select * from works where composer=X ORDER BY shortname" I get the records in perfect order.

However, when I do "ORDER BY cataloga,shortname" it...

Weird indexation problem

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