have had the same font size issues in suse tumbleweed linux , although true of some other linux installs …
in that
where kde apps tend to cope and correct there font sizes and are usable , but not so for gtk apps , (system wide not just unreal engine that is gtk , include chrome , nvidia desktop tray and others )
if you change the fonts from within the app it only affects the one app (if it lets you )
if you change the dpi system wide then it affects all apps so kde apps emd up with masive fonts and gtk aps to normal size
anyway the way to solve it is to set the global scale to 150% in the display settings (this also corrects icon size )
then ensure the kde font size is reduced back to 96dpi
this resolves the issue system wide so also ensures other gtk apps also conform
(you may have to revert the changes made to individual apps back to there defaults if you changed them to try to resolve this )
far easyer than trying to set the fonts individualy for each app