Hi Folks, On a Dell Latitude with Intel graphics, running gnome flashback, I also saw this behavior after upgrading from stretch to buster. This laptop, however, is running gdm3, not lightdm. I am seeing the exact same symptoms, with VT switching generally working around the issue post-lock.
So far, the workaround at https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=929834#66 has resolved this for me. I will update this bug if I see any further issues. John