Thank you. What is happening here is unstable being unstable. The Qt team is currently performing a transition of qtbase-abi and the package qt5ct has yet to be rebuilt with the new qt5 abi version. Progress is tracked here: Transition: qtbase-opensource-src and qtdeclarative-opensource-src
So this should sort itself out by itself once all packages get rebuilt with the new qt base abi. At that point, qt5ct will no longer Depend on qtbase-abi-5-15-15 which is no longer provided by libqt5core5t64 in unstable. Instead it will depend on qtbase-abi-5-15-17 and then be installable again.
That is similar, yes.