I don't know about that.
I honestly don't know if it is a flag. There are other reasons for not investing in real estate in a plan, starting with 1) valuation - what is it worth, each year? and 2) real estate often involves financing, which can trigger Unrelated Business Taxable Income, and 3) in small plans, owners often blur the lines between the plan and their personal investments, and don't understand that they can't buy/sell from/to themselves or a family member, or invest jointly with same, and otherwise gum things up by paying taxes and other expenses themselves when the plan should be paying, etc. etc. As you note, it is clearly "legal."