Polish characters of Terminus font not rendered properly

I've created a very simple Qt application (source code available on gist). It creates a few buttons, each with the same text (normal characters in the first line, polish characters in the second line). Each button has Terminus font of different size.

When I just run the program, the polish characters are rendered using different font (probably some default one) for some of the sizes.
When I set LANG variable to "pl" and run the program, all characters are rendered properly.
When I change dpi from 96 (default) to something else, e.g. 97, and run the program, all characters are rendered properly.

I checked this on two machines: Debian Testing (Qt 4.7.3) and Ubuntu Lucid (Qt 4.6.2). The behaviour is the same, screenshots are in attachments.

Debian details:

]$ lsb_release -a
LSB Version: core-2.0-amd64:core-2.0-noarch:core-3.0-amd64:core-3.0-noarch:core-3.1-amd64:core-3.1-noarch:core-3.2-amd64:core-3.2-noarch
Distributor ID: Debian
Description: Debian GNU/Linux testing (wheezy)
Release: testing
Codename: wheezy

]$ uname -a
Linux pcuj 3.0.0-1-amd64 #1 SMP Sat Aug 27 16:21:11 UTC 2011 x86_64 GNU/Linux

]$ konsole -version
Qt: 4.7.3
KDE Development Platform: 4.6.5 (4.6.5)
Konsole: 2.6.4

]$ locale
LANG=en_US.UTF-8
LANGUAGE=
LC_CTYPE="en_US.UTF-8"
LC_NUMERIC="en_US.UTF-8"
LC_TIME="en_US.UTF-8"
LC_COLLATE="en_US.UTF-8"
LC_MONETARY="en_US.UTF-8"
LC_MESSAGES="en_US.UTF-8"
LC_PAPER="en_US.UTF-8"
LC_NAME="en_US.UTF-8"
LC_ADDRESS="en_US.UTF-8"
LC_TELEPHONE="en_US.UTF-8"
LC_MEASUREMENT="en_US.UTF-8"
LC_IDENTIFICATION="en_US.UTF-8"
LC_ALL=

]$ aptitude show xfonts-terminus | grep Version
Version: 4.30-2

]$ aptitude show libfreetype6 | grep Version
Version: 2.4.7-2

]$ aptitude show fontconfig libfontconfig1 | grep Version
Version: 2.8.0-3
Version: 2.8.0-3

Ubuntu details:

]$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 10.04.3 LTS
Release: 10.04
Codename: lucid

]$ uname -a
Linux zabawa 2.6.32-33-generic #72-Ubuntu SMP Fri Jul 29 21:07:13 UTC 2011 x86_64 GNU/Linux

]$ konsole -version
Qt: 4.6.2
KDE Development Platform: 4.4.5 (KDE 4.4.5)
Konsole: 2.4.5

]$ locale
LANG=en_US.UTF-8
LANGUAGE=
LC_CTYPE="en_US.UTF-8"
LC_NUMERIC="en_US.UTF-8"
LC_TIME="en_US.UTF-8"
LC_COLLATE="en_US.UTF-8"
LC_MONETARY="en_US.UTF-8"
LC_MESSAGES="en_US.UTF-8"
LC_PAPER="en_US.UTF-8"
LC_NAME="en_US.UTF-8"
LC_ADDRESS="en_US.UTF-8"
LC_TELEPHONE="en_US.UTF-8"
LC_MEASUREMENT="en_US.UTF-8"
LC_IDENTIFICATION="en_US.UTF-8"
LC_ALL=

]$ aptitude show xfonts-terminus | grep Version
Version: 4.30-2

]$ aptitude show libfreetype6 | grep Version
Version: 2.3.11-1ubuntu2.4

]$ aptitude show fontconfig libfontconfig1 | grep Version
Version: 2.8.0-2ubuntu1
Version: 2.8.0-2ubuntu1