As kamenges said above, it's likely that there are components, such as VFD's or Power Supplies that have line filtering to ground, and the components are not meant to float above the nominal 277v. I've seen many failures of these filters on 480v delta systems, whether corner grounded or open. Even with an impedance ground system, the filters are not happy on delta power.
I recently had a customer with an older plant that had NO Grounds run to control cabinets, and had numerous faults on the power supplies and drives after an upgrade. The older stuff didn't mind, but the newer components were much more finicky.