It's network/datovka in OBS. I have:
%if 0%{qt_version} == 6
# private-devel is only temporary to fix "Library 'libatomic' is not defined."
#BuildRequires:  qt6-base-private-devel
BuildRequires:  qt6-tools-linguist
%else
BuildRequires:  libqt5-linguist
%endif
BuildRequires:  openssl-devel
BuildRequires:  pkgconfig
BuildRequires:  cmake(Qt%{qt_version}Core) >= %{qt_version_full}
BuildRequires:  cmake(Qt%{qt_version}Gui) >= %{qt_version_full}
BuildRequires:  cmake(Qt%{qt_version}Network) >= %{qt_version_full}
BuildRequires:  cmake(Qt%{qt_version}PrintSupport) >= %{qt_version_full}
BuildRequires:  cmake(Qt%{qt_version}Sql) >= %{qt_version_full}
BuildRequires:  cmake(Qt%{qt_version}Svg) >= %{qt_version_full}
BuildRequires:  cmake(Qt%{qt_version}Widgets) >= %{qt_version_full}
BuildRequires:  pkgconfig(libdatovka) >= 0.1.2


And the build fails with:
> [    4s] + /usr/lib64/qt6/bin/qmake 'QMAKE_CXXFLAGS=-std=c++17 -O2 -Wall -D_FORTIFY_SOURCE=2 -fstack-protector-strong -funwind-tables -fasynchronous-unwind-tables -fstack-clash-protection -Werror=return-type -flto=auto -O2 -Wall -D_FORTIFY_SOURCE=2 -fstack-protector-strong -funwind-tables -fasynchronous-unwind-tables -fstack-clash-protection -Werror=return-type -flto=auto' 'QMAKE_CFLAGS=-O2 -Wall -D_FORTIFY_SOURCE=2 -fstack-protector-strong -funwind-tables -fasynchronous-unwind-tables -fstack-clash-protection -Werror=return-type -flto=auto -O2 -Wall -D_FORTIFY_SOURCE=2 -fstack-protector-strong -funwind-tables -fasynchronous-unwind-tables -fstack-clash-protection -Werror=return-type -flto=auto' 'QMAKE_LFLAGS= -Wl,--as-needed -Wl,--no-undefined' PREFIX=/usr DISABLE_VERSION_CHECK_BY_DEFAULT=1
> [    4s] Info: creating stash file /home/abuild/rpmbuild/BUILD/datovka-4.19.0/.qmake.stash
> [    4s] Project WARNING: Disabling version check by default.
> [    4s] Project ERROR: Library 'libatomic' is not defined.

I have to uncomment the first require to fix this: qt6-base-private-devel

Then, these additional packages are installed:
> cups-config-2.3.3op2-8.1
> cups-devel-2.3.3op2-8.1
> cups-rpm-helper-2.0.1-1.8
> fontconfig-devel-2.13.1-3.1
> freetype2-devel-2.11.1-1.2
> libavahi-client3-0.8-12.1
> libavahi-common3-0.8-12.1
> libbrotli-devel-1.0.9-1.6
> libbrotlienc1-1.0.9-1.6
> libbz2-devel-1.0.8-3.4
> libcupsimage2-2.3.3op2-8.1
> libcups2-2.3.3op2-8.1
> libexpat-devel-2.4.3-1.1
> libgnutls30-3.7.3-1.1
> libhogweed6-3.7.3-2.1
> libinput-devel-1.19.3-2.1
> libnettle8-3.7.3-2.1
> libp11-kit0-0.23.22-4.1
> libtasn1-6-4.18.0-1.2
> libudev-devel-249.9-1.1
> libuuid-devel-2.37.3-1.1
> libxkbcommon-x11-devel-1.3.1-1.4
> mtdev-devel-1.1.6-1.6
> python38-pycups-2.0.1-1.8
> qt6-base-private-devel-6.2.2-2.2
> qt6-core-private-devel-6.2.2-2.2
> qt6-dbus-private-devel-6.2.2-2.2
> qt6-gui-private-devel-6.2.2-2.2
> qt6-kmssupport-devel-static-6.2.2-2.2
> qt6-kmssupport-private-devel-6.2.2-2.2
> qt6-network-private-devel-6.2.2-2.2
> qt6-opengl-private-devel-6.2.2-2.2
> qt6-platformsupport-devel-static-6.2.2-2.2
> qt6-platformsupport-private-devel-6.2.2-2.2
> qt6-printsupport-private-devel-6.2.2-2.2
> qt6-sql-private-devel-6.2.2-2.2
> qt6-test-private-devel-6.2.2-2.2
> qt6-widgets-private-devel-6.2.2-2.2
> qt6-xml-private-devel-6.2.2-2.2
> system-user-lp-20170617-24.3
> tslib-devel-1.22-1.6
> tslib-plugins-1.22-1.6
> tslib-1.22-1.6

And the build proceeds fine. It's likely some dependency missing in some devel
package.

This is only a qt6 problem, qt5 does not exhibit it.