Camera switching draws black on sides


Hello everyone, new to UE4 user here, and for another question.
I want to create switching camera between
a) player walking around the environment (usual first person thing)
b) player solving puzzles and riddles

so I tried doing this way, and while everything works as intended - I see a weird black line suddenly drawn and slowly fades away until camera transition is finished.

This I didn’t see in other tutorial videos and articles.

Has anyone got idea why this is happening? Thanks