Hmm… having to troubleshoot this kind of incidents, let alone fix them can cause significant stress.
Qt is a programming language currently at version 6 and that means Qt 5 is in the process of being deprecated, just like Plasma 6 replaced Plasma 5.
Such package will no longer be maintained, but the recipe (the PKGBUILD) is moved to AUR.
A long time ago I wrote a short document [root tip] [How To] Avoid common pitfalls as developer.
The document linked above 
regarding AUR serve a similar purpose although it goes in-depth to explain why custom packages need special attention.