Contrary to apt, aptitude refuses to upgrade packages related to qt6.
So, to help aptitude, I've provided a command line with the packages
to upgrade and to remove (listed by apt). But this doesn't solve the
issue:

cventin:~> aptitude install gir1.2-poppler-0.18 libpoppler-glib8t64 libpoppler-qt6-3t64 libqt6multimedia6 libqt6sql6-sqlite libqt6wlshellintegration6 libwsutil15t64 qpdfview-pdf-poppler-plugin qt6-wayland libpoppler-cpp0t64 libpoppler-glib8t64-dbgsym libpoppler134 libqt6qml6 libqt6svg6 libwireshark-data poppler-utils qpdfview-ps-plugin wireshark libpoppler-dev libpoppler-private-dev libqt6core5compat6 libqt6qmlmodels6 libqt6waylandclient6 libwireshark17t64 qpdfview qt6-gtk-platformtheme wireshark-common libpoppler-glib-dev libpoppler-qt5-1t64 libqt6core6t64 libqt6quick6 libqt6waylandcompositor6 libwiretap14t64 qpdfview-djvu-plugin qt6-qpa-plugins libqt6dbus6t64- libqt6gui6t64- libqt6network6t64- libqt6opengl6t64- libqt6printsupport6t64- libqt6sql6t64- libqt6waylandeglclienthwintegration6- libqt6waylandeglcompositorhwintegration6- libqt6widgets6t64- -s
qt6-wayland is already installed at the requested version (6.4.2-5+b2)
libqt6waylandclient6 is already installed at the requested version (6.4.2-5+b2)
libqt6quick6 is already installed at the requested version (6.4.2+dfsg-4+b2)
libqt6waylandcompositor6 is already installed at the requested version (6.4.2-5+b2)
The following NEW packages will be installed:
  libqt6network6{ab} libqt6opengl6{ab} libqt6sql6{a}
The following packages will be REMOVED:
  libqt6dbus6t64 libqt6gui6t64 libqt6network6t64 libqt6opengl6t64{u}
  libqt6printsupport6t64 libqt6qmlmodels6{u} libqt6quick6{u} libqt6sql6t64
  libqt6waylandclient6{u} libqt6waylandcompositor6{u}
  libqt6waylandeglclienthwintegration6{u}
  libqt6waylandeglcompositorhwintegration6{u} libqt6widgets6t64
  qt6-gtk-platformtheme{u} qt6-qpa-plugins{u} qt6-wayland{u}
The following packages will be upgraded:
  gir1.2-poppler-0.18 libpoppler-cpp0t64 libpoppler-dev libpoppler-glib-dev
  libpoppler-glib8t64 libpoppler-glib8t64-dbgsym libpoppler-private-dev
  libpoppler-qt5-1t64 libpoppler-qt6-3t64{b} libpoppler134
  libqt6core5compat6 libqt6core6t64 libqt6multimedia6{b} libqt6qml6
  libqt6sql6-sqlite libqt6svg6{b} libqt6wlshellintegration6{b}
  libwireshark-data libwireshark17t64 libwiretap14t64 libwsutil15t64
  poppler-utils qpdfview{b} qpdfview-djvu-plugin{b}
  qpdfview-pdf-poppler-plugin{b} qpdfview-ps-plugin{b} wireshark{b}
  wireshark-common wireshark-doc
29 packages upgraded, 3 newly installed, 16 to remove and 113 not upgraded.
Need to get 47.0 MB of archives. After unpacking 32.0 MB will be freed.
The following packages have unmet dependencies:
[...]

The first 4 lines are wrong:

qt6-wayland is already installed at the requested version (6.4.2-5+b2)
libqt6waylandclient6 is already installed at the requested version (6.4.2-5+b2)
libqt6quick6 is already installed at the requested version (6.4.2+dfsg-4+b2)
libqt6waylandcompositor6 is already installed at the requested version (6.4.2-5+b2)

For instance, for qt6-wayland, I did not request 6.4.2-5+b2, but
unstable.

cventin:~> apt-show-versions -a qt6-wayland
qt6-wayland:amd64 6.4.2-5+b2 install ok installed
qt6-wayland:amd64 6.4.2-1 stable   ftp.debian.org
No stable-updates version
qt6-wayland:amd64 6.6.2-2 testing  ftp.debian.org
qt6-wayland:amd64 6.6.2-2 unstable ftp.debian.org
No experimental version
qt6-wayland:amd64/testing 6.4.2-5+b2 upgradeable to 6.6.2-2
qt6-wayland:i386 6.4.2-1 stable   ftp.debian.org
No stable-updates version
qt6-wayland:i386 6.6.2-2 testing  ftp.debian.org
qt6-wayland:i386 6.6.2-2 unstable ftp.debian.org
No experimental version
qt6-wayland:i386 not installed

and unstable is 6.6.2-2, not 6.4.2-5+b2!

However, after some proposed solutions with umet dependencies, there is an
acceptable one:

The following actions will resolve these dependencies:

      Install the following packages:
1)      libqt6dbus6 [6.6.2+dfsg-8 (testing, unstable)]
2)      libqt6gui6 [6.6.2+dfsg-8 (testing, unstable)]
3)      libqt6printsupport6 [6.6.2+dfsg-8 (testing, unstable)]
4)      libqt6widgets6 [6.6.2+dfsg-8 (testing, unstable)]
5)      libxcb-cursor0 [0.1.4-1+b1 (testing, unstable)]

      Upgrade the following packages:
6)      libqt6qmlmodels6 [6.4.2+dfsg-4+b2 (now) -> 6.6.2+dfsg-3 (testing, unstab
7)      libqt6quick6 [6.4.2+dfsg-4+b2 (now) -> 6.6.2+dfsg-3 (testing, unstable)]
8)      libqt6waylandclient6 [6.4.2-5+b2 (now) -> 6.6.2-2 (testing, unstable)]
9)      libqt6waylandcompositor6 [6.4.2-5+b2 (now) -> 6.6.2-2 (testing, unstable
10)     qt6-gtk-platformtheme [6.4.2+dfsg-21.1+b1 (now) -> 6.6.2+dfsg-8 (testing
11)     qt6-qpa-plugins [6.4.2+dfsg-21.1+b1 (now) -> 6.6.2+dfsg-8 (testing, unst
12)     qt6-wayland [6.4.2-5+b2 (now) -> 6.6.2-2 (testing, unstable)]

But it should have been the initial solution.