Why would a lender be troubled by this? The property, due to a new source of income, is worth more. Seems like more income = less risk from the lender’s point of view.
Fannie Mae and Freddie Mac have extensive rules on what mortgages they'll buy. These are known as conforming loans and get buyers the best terms and easiest approval (thus raising sale prices). For condos these rules are especially complex. Any property which has commercial use or rentals is already flagged for extra scrutiny and there are some uses which flat-out get it disqualified.
Lenders not only want to be able to have Fannie/Freddie buy their loans eventually, but they also want the home to be easily sold. And traditionally home buyers want the "white picket fence" dream. Once the property turns into a an apartment/vacation/commercial complex, their market of buyers totally changes. There's nothing wrong with that inherently but it means that different people need to be underwriting and making those loans who can properly assess the value.
