Fractional scaling was initially an afterthought for Wayland and its implementations, like in Plasma, lagged about adding it and then implementing it without bugs. Fractional scaling was broken on Plasma for a bit because of it, and GNOME decided that users didn't need it. That hasn't been the case for a while, now, and it works on both desktops.

Xorg also had to implement it, and at one point Xorg's implementation was more stable than what popular Wayland compositors offered.