The problem is that Cubase runs in desktop mode with accelerated graphics, which means that all the parts of graphics card / drivers / windows display manager / window composition / monitor must align “just so” for it to work out well.
Also, if you have two monitors, a single program displaying across both monitors may be forced into a particular mode (which may be a fallback mode, rather than the mode of one of the two monitors.)
Presumably, something about your chain with the new monitor wiggled that setup such that it no longer does what you desire. Is this the only monitor you have attached, or have you attached more monitors? Is the monitor “g-sync” capable/verified? There are tons more hidden attributes that affect whether this will “work” or not, and I don’t know how to check for them – basically, for anything not-full-screen, I’ve given up :-/