Arch is good for that:
https://archive.archlinux.org/packages/q (list of all previous edition pkgs starting with q)
For example:
cd /tmp
% wget [url]https://archive.archlinux.org/packages/q/qt5-3d/qt5-3d-5.14.1-2-x86_64.pkg.tar.zst[/url]
% sudo pacman -U qt5-3d-5.14.1-2-x86_64.pkg.tar.zst
Make sure that all packages you have that are 5.14.2 ... have to be replaced with 5.14.1 versions
To find out what you updated you can look at /var/log/pacman.log (go to the very bottom and cut/paste the ones that begin with qt5-) or
"% cat /var/log/pacman.log | grep "qt5-"
Alternatively you can wait for eric or jean-michel to make and send the new qt5-base pkg on the repository. Probably later tonight (GMT)
If you make a list and save it into a file you can temporarily add them into an "Ignore = qt5-...... " line in pacman.conf and keep the list for future occurrences of this trouble. Adding arch's version qt5-base may give you trouble because it depends on systemd, and I suspect if the display manager is qt5 dependent it will fail without it.
But I suspect you have to do this for 12-15 packages ...