When I use gbak with "nohup" in Debian, it seems gbak's output to nohup.out is written in chunks,

nohup gbak -b -v -g database database.fbk &

It shows, for instance:

gbak:520000 records written
gbak:530000 re

And after many seconds or minutes, it writes another chunk to the log.
This behavior appeared in Firebird version 3, it worked as expected in Firebird version 2.5.
When the output is to the console, it works as expected.