On a fresh stretch install this bug isn't present, while I got it on older installations of stretch and sid, upgraded gradually from Plasma 4.
Maybe the problem is triggered by some old configuration file? I noticed that there are 2 konsole profiles in the options, both named "Shell" and even if you delete one of them, they keep reappearing. Also, the konsole window doesn't seem to remain in maximized state.
Nothing of this happens on a fresh installation.
I hope this helps.