I found that '+' and '-' icon are replaced by ban icon, but they still working.
My system is (semi-)rolling with xfce.
Does anybody know what's happened? Thanks.
Did you try different icon theme and/or different gtk theme?
I didn't change the icon theme, but I tried to change it after I saw your post.
And I found that only them Tela and Tela dark didn't have '+' and '-' icon, but other theme, like Ultra-Flat-Icons, the '+' and '-' appear.
There has been new Tela icons release. Hopefully this issue will be fixed.
Edit. Pavroo just uploaded newest release. Update your system and give Tela icons another try.