I see this under wayland, and can reproduce it under X as well.
Using the egl renderer when I toggle to fullscreen in terminology with f11, as many as 3 frames are rendered - one that is unchanged from the unfullscreen state, one in which the content is in the top left corner and the rest is dark grey, and finally a correct frame.
I can see this readily under weston and E if I attach a debugger to terminology and break on eglSwapBuffersWithDamageEXT, or under X if I break on eglSwapBuffers. I haven't tried with glx...
It's also easiest if terminology is run -c 0 so the cursor blinking doesn't confound reproduction.