I've the same bug.After downgrade libkdecorations2-5v5:amd64 to 4:5.17.5-2, libkf5screen-bin:amd64 to 4:5.17.5-3 and libkf5screen7:amd64 to 4:5.17.5-3 i.e. to the previous version, kde works well.
I do'nt know more precisely if only one package is responsible for the problem.
Regards -- Francois Mescam