You said you were on KDE, I have the same issue, which this post was helpful to find the root cause.
xrdb -query -all | grep -i emacs, revealed some entries for Emacs which I didn't set myself:
$ xrdb -query -all | grep -i emacs
Searching the disk for the files defining these entries I found:
$ grep -Fnr 'Emacs*XlwScrollBar' /etc /usr/share 2>/dev/null
By searching about this file
/usr/share/kdisplay/app-defaults/Emacs.ad, I found this old but relevant thread on the Emacs mailing list:
Apparently KDE sets some X resources for applications like Emacs,
so that the application look'n'feel matches the KDE theme.
This thread suggests to remove only the attributes that aren't desired but to keep others so the look'n'feel stays close to the KDE theme:
In this way, KDE still changes the colors/fonts of other applications
AND also some things in Emacs (like the Menubar for example, which matches the KDE-menus colors/font).
Note: I could not find a difference in the look'n'feel after commenting the whole file, so it's possible that this is no longer needed even for Menubar and other graphical elements that the resource file sets.
My issue was just the cursor color, which is solved by commenting
! breaks Emacs cursor color
! Emacs*Foreground: WINDOW_FOREGROUND