Not a problem as long as the current ratings are sufficient for your needs, but once I got deceived by equipment used this way. I designed a PV system for a 480/277V service that turned out to be 208/120V. Lesson learned: ALWAYS check the voltage; don't just look at the equipment nameplates.
I feel your pain. Your initial site visit team is probably not equipped to take panels apart and work energized. Then you plan everything on the assumption of 480V with no data to tell you the actual situation. Also frustrating when a dead front covers the identity of a breaker, so you can't investigate its specs in more detail.
Seems like a situation that should require field-installed markings.
"If distribution equipment has a nameplate voltage rating exceeding 120% of the largest nominal voltage rating present within the system, the enclosure shall be field-labeled with the nominal voltage of the system, in a location visible without exposing live parts."
The 120% number allows the status quo for 208V systems using 240V equipment, where other clues likely tell you this information. Probably a good idea to label it anyhow.