When it comes to fixed income assets, whether rental real estate or government bonds, it's unusual for highly-leveraged assets to yield less than the same asset unleveraged or lowly-leveraged. This is especially so in countries where interest costs are tax deductible.
If we exclude capital losses (i.e. the property sells in future at a price less than it was purchased) or net rental income that doesn't keep up with maintenance, regulatory, taxation, inflation and / or other costs, there is one primary scenario where higher leverage results in lower yields compared to lower leverage, even if rental income keeps up with non-funding costs. This occurs when variable rate financing is used and rates substantially increase.
Borrowers and lenders in different countries have different mortgage rate customs. Some are more likely to have long-term fixed rates; some prefer variable rates; and others are a hybrid, i.e. fixed for a few years and then become variable.
If variable rates are used for a mortgage and the reference rates increase substantially, as they did in the US during the 1970s, the borrower can easily become "upside-down," i.e. owe more on the mortgage than the property is then worth, and have mortgage service costs that exceed the net rental income. Some of those costs aren't easy to pass along to renters, even when there are periodic lease renewals or base rent increases referencing inflation rates. Central banks set policies for what would be the lowest short-term rates in a country that has such a bank. Private sector rates are established broadly by supply and demand for credit and can thus diverge markedly from central bank rates.
Over time, the higher finance-carrying-cost-to-net-rental-income ratio should abate as (1) rental market prices change to reflect the costs and (2) the landlord can reinvest his net rental income at a higher rate. In the short-term though, this can result in the landlord having to "eat" the costs making his yield on his leveraged fixed income asset less than what he would have without leverage, even if the property was later sold at same price regardless of financing method.
Interestingly, and on the flip side, this is one of the quirks in finance where an accounting liability can become, at least in part, an economic asset. If a landlord borrows at a high loan-to-value ratio for a fixed interest rate for the life of the mortgage and rates, variable and fixed, were to increase substantially, the difference between his original rate and the present rates accrues to him.
If he's able to sell the property with the loan attached (which is not uncommon for commercial, industrial and sometimes municipal real estate), the buyer will be assuming a liability with a lower carrying cost than his present alternatives and will hence pay a higher price for the property than if it were unleveraged.
With long-term rates in many economically advanced countries at historic lows, if a borrower today were to take a long-term fixed rate loan and rates shortly after increased substantially, he may have an instant profit in this scenario even if his property hasn't increased in value.