In the past, when it wasn't exposed in GUI, and worked for Wayland only, it was necessary to logout and login between enabling fractional scaling and setting the scale itself (the reason was, that Xwayland was started in different mode then; technicality, that doesn't matter no normal users, but then, the functionality was labeled experimental and enabled via gsettings anyway).
The X11 fractional scaling is a different beast; it uses xrandr scaling and requires cooperation from the graphic card (the output encoder, specifically - it sets up framebuffer at different size than the output to display). That also means, that it won't work on machines that do not have output encoder at all and assume framebuffer resolution is equal to output resolution, like remote sessions or virtual machine clients. Here I agree, that some error message would be appreciated. It was also brave from Canonical to enable it for LTS, without going through a testing cycle in a non-LTS release.
If it really was intentional, a small dialog or log that explains _why_ I couldn't scale monitors differently would have sufficed, or, simply moving the percentage adjustment control out of the settings for each display.
On that note, 20.04 is supposed to support fractional scaling, but if I try to turn the toggle on, it just fails silently, so are we back to square one, or did a developer somewhere decide some precondition means I can't _currently_ turn it on, but didn't feel it was worth telling me, the user, why?