I see the problem, this SP was called by a trigger, I have added
returning_values clause and the problem has been solved.
I thought that the returning_values was an optional clause and if you
do not need the returning, you could avoid it.

In any case, in my opinion, it is a bug of Firebird, it must validate
the changes in the database. FB cannot allows to corrupt the database.
I have discovered it just when I needed to restore a database, and I
have not been able to restore it.

In any case, thank you.