Why is engine light on before refueling with diesel in a rental f350 SD truck

What is your question?

Noted Engine light came on within few days of rental despite no attempt to refuel. But rental company claimed the light came on due to refueling with wrong fuel. I am absolutely sure (100%) that I refueled on last day of rental with diesel and that the engine light had come on several days before refueling.

What makes this problem better or worse?

No idea. The light stayed on until return of vehicle

How long has your 2018 Ford F-350 Super Duty had this problem?

A few days into rental. Possible it was on earlier but did not notice.