Hot water issues in the apartment and plumbers are stumped?
Very odd, but here's a thought on how to troubleshoot. Shut off water pressure into your water heater. Test each one of your hot water taps. If any of them work, something is mixing cold water into your hot water pipes. Some one-handle faucet and shower fixtures can do this if their internal seals are worn out. Find it and fix it. If they all only work when water pressure is supplied to your water heater, then the problem is likely the water heater itself.

Are the other units on the same piping? If so, the mix may be caused by a recent change in their plumbing. Maybe you can engage them in the troubleshooting.