According to this commit https://invent.kde.org/plasma/kwin/-/commit/28ed0ced1741ec409716ba506446371ee2b25351 kwayland-server was merged into kwin with version 5.25. When upgrading to this version, resolving dependencies fail with [blocks B ] kde-plasma/kwayland-server ("kde-plasma/kwayland-server" is soft blocking kde-plasma/kwin-5.25.5) and (kde-plasma/kwayland-server-5.24.6:5/5::gentoo, ebuild scheduled for merge) pulled in by kde-plasma/kwayland-server:5 required by @kde-plasma Commenting kde-plasma/kwayland-server in /var/db/repos/kde/sets/kde-plasma resolved the issue and allowed me to start update process.
Unversioned sets in kde overlay are for stable versions. KDE Plasma 5.25 is not yet stable, so the set is based on 5.24.
Thanks and sorry for wasting your time.
No worries, that fact is not necessarily obvious to the user.
That said; stabilisation of 5.25.5 was just started in bug 868909 which means the set will be regenerated for the future most recent stable Plasma version very soon.