What is behind the scene?
It seems at the database creation the GLOBAL_NAME setting for this database is created as well based on db_name and db_domain settings. Any changes to those base parameter are not reflected further in this GLOBAL_NAME value. And that is GLOBAL_NAME which influence database links name - its domain part is added to every database link.
How to deal with it?
http://dbatips4u.blogspot.com/2009/04/tip21-db-link-name-suffix.html provides the details.
I add here small note to it - I was especially interested in cancelling the global_name domain part at all, which is less trivial than just setting it to something new as the simple ALTER DATABASE RENAME GLOBAL_NAME TO "{db_name}"; does not work as planned (the domain part stays in place). I followed the notes from the link above. I run UPDATE GLOBAL_NAME SET GLOBAL_NAME='
The important part was a database restart, after which all started work as planned.
Btw. our GLOBAL_NAMES was set to FALSE, which is different from the settings presented in the tip from the link above - may be that is the clue, why I needed to restart.