Skewed / angled default viewport?

Hi all,

I switched from 5.1 to 5.3.1 but now my viewport is acting up…

Every time I switch the viewport type from Cinematic Viewport to Default Viewport it ends up looking skewed / angled. When I hold RMB and try to move around the view remains angled.

The only way to fix it is to play the level, exit, and the viewport returns to normal.

Am I doing something wrong? What causing this weird viewport behaviour?