Then it's hard to say what's wrong. You can try to run flatpak run --command=sh org.kde.Platform and see if you get the prompt inside flatpak. It could be a manifestation of using old kernel, but I am speculating here. Note that neochapay started working on Qt 5.12 builds for Nemo. If he pulls it off, assuming that we can install it in parallel with Qt 5.6, we can probably run 5.12 apps via similar solution as Flatpak Runner even without Silica support. Maybe its more realistic to wait for this solution than trying to fight requirements with old kernels.
FATAL: kernel too old error: ldconfig failed, exit status 34304
bwrap: execvp xdg-dbus-proxy: No such file or directory