> I guess that this is a bug; is it known?

Yes, but this is a bug in all InterBase <= 6.0 and Firebird < 2.0, so you
can call it a feature. It is fixed in FB 2.0 with new INTL stuff.

> What is the explanation?

Engine incorrectly treats the multi-byte character sets and pads the column
to the byte length, not the character length. Unfortunately that was not
possible to fix before new INTL was added.

> Is there a connection to the Unicode character set?

It won't help with FB < 2.0

Roman