What is the 12 V buck-boost rated for? There certainly exist 24->12 V supplies that can handle that much load. Is this a commercial supply? Was it designed to go with this 3D printer or did you just use a random supply you had around? Have you modified the printer? If the power supply is part of a system designed to work together it should be rated high enough. It might be that something in the controller is broken and drawing a large fault current causing the voltage drop.
It seems unlikely that it is a problem with your 24V supply but you should definitely measure the 24 V rail with everything on to see if it is dropping a lot.