R. Kelly was present during the Isley Brothers' musical showdown with Earth, Wind & Fire -- or he was in spirit anyway, because the disgraced singer's music was part of the IB's set.

Ron and Ernie Isley faced off against three members of EW&F Sunday night in a highly anticipated "Verzuz" battle -- with none other than Steve Harvey hosting in an extravagant Easter getup.

Everything was moving along smoothly, with each group taking turns back and forth to perform some of their greatest hits. It was great ... that is, until Ron and Ernie decided to start crooning some of their later tracks from the 2000s, including some R. Kelly tracks he either produced/wrote or was featured in, like "Contagious" and "Busted."

They did "Contagious" first ... which a lot of folks watching from home were NOT happy about. Thankfully, the duo had the wisdom not to include R. Kelly's featured verse, which is sprinkled in toward the end. Still, the guy wrote and produced the whole thing.

Ditto for "Busted," which didn't feature R. Kelly's vocals directly ... but was also written and produced by the guy. Fans felt it wasn't a good look ... for obvious reasons.

Remember, R. Kelly is currently in prison awaiting trial on several different criminal charges -- both federal and state ... which involve sexual assault, sexual trafficking, abuse of a minor, racketeering, obstruction of justice and more. He's pled not guilty to all of the charges.

It's clear he doesn't have much goodwill left among music lovers, past or present -- the only question ... do the Isleys care??? They seemed pretty comfy up there with the Kels tracks.