weird minimise/maximise/close icons after switch from gnome > kde > gnome

so i decided to try out KDE as gnome wasn't doing everything i wanted, however KDE didn't work out for me either so i switched back to gnome, completing an uninstall and purge of the KDE components.

I used gnome tweaks to restore colours/themes, and that all looks right, however i've noticed some weird icons being used for the minimise/maximise/close buttons in some windows.

I've included a screenshot below showing it affecting some apps but not others - from top to bottom:

• remmina (affected)
• pop shop (affected)
• sublime (unaffected)
• gnome tweaks (unaffected, also shows my appearance settings)

edit: it's worth noting that if i toggle the 'Legacy Applications' setting from Pop-dark to pop, and then back to Pop-dark, it fixes the issue - until i logout or reboot.

https://imgur.com/a/ZqjF16z

Is anyone able to point what is causing this / how to restore the defaults please?