Buying single family home zoned as multi unit

We are under contract for a SFH in Chicago. But during attorney review it was found that the sellers bought it 5 years ago when it was 3 units, and converted it to one big SFH. They got permits for electrical, plumbing, and moving walls, but never got a deconversion permit. We're told they probably never did this because it would raise property taxes. Our appraisal came back as a SFH and we are cleared to close. We are fine leaving the zoning alone, but our only concern is that if in the future we have to get a deconversion permit, or the city finds out we have a SFH where a MU is zoned, will we have to pay back taxes? The lot is zoned as RS-3. Thank you!