Where are the speakers located (if any)? Magnetic field from speakers can (and usually does) cause the electron beam to deviate causing this behavior.

Also monitors have a Degauss function in their OSD menu. Run it, and see if it helps. Unless you already did it and described it as "resetting" the monitor.

If nothing above helps (no speakers anywhere near or anything with magnetic field or degauss does not do anything) you may need to return the monitor for a warranty replacement.