It's a common to have Network (remote:/) bookmark in KDE file dialogs. And it's a common to run new KF5-based applications in KDE Workspace 4. But Network bookmark in KF5-based applications doesn't work because kio_remote is provided by Plasma Workspace 5. Perhaps kio_remote should be moved from Plasma Workspace 5 to KF5 KIO. Reproducible: Always
Kde4 is long gone; this isn't worth the effort to fix (if it's even feasible at all).