I think it might be possible to work around the uscan bug by only having the URL for stable releases in the watch file. This means you no longer track the unstable releases but it should almost certainly work.
You may also be able to track both stable and unstable releases with 1 URL:
It should match both stable and unstable, though I have no way to test if this actually works. It seems like uscan does still only return 1 version for a single URL so if it works, this should return the latest version, either stable or unstable.
As mentioned before, I think this problem may affect many other KDE packages, mostly from Plasma and Frameworks. The KDE Applications seem to be fine since most of those only seem to track the stable URL.