@TaxGuyBill answered a question like this back in 2019 on a TurboTax forum.  I'm too lazy to look up the source or figure out how to post a link, but what he said:

"The determining factor is that if 80% of the total building rents are for Residential Dwelling Units, then the entire building and improvements are deprecated over 27.5 years.  If it does not meet the 80% test, the entire building and improvements are depreciated over 39 years."