Why shouldn't it be a clean kdenext installation? How comes it is not a clean kdenext installation? How can one get a "clean" kdenext installation at all? Of course I have kde packages installed from siduction fixes simply because they are needed but not available in kdenext (like kde-window-manager, kde-style-oxygen, kde-workspace-bin, etcpp). Those packages are not the problem. It's kdenext packages that would get removed, the kdenext packages won't accept libical1a (or vice-versa) if I would want to install it.
It is not the d-u that wants to remove the kdenext packages. The system is "happy" with the installed libical1, nothing is hold, nothing shall be removed, it's as if it doesn't know that there is a libical1a. I wouldn't have noticed that there might be a problem (i.e. a package that is installed but not available anymore) if I haven't read about in the forum. The kdenext packages would be removed if I would try to install the available libical1a manually. I don't, the d-u doesn't either.