It has come to my attention that today's patch has also broken custom chairs. I am currently investigating the cause and when I know the issues, I will update all my chairs (oh god i have too many chairs).
Update: Turns out chairs have been restricted to 32bit instances which is not ideal. I wont be updating until the gurus investigate and decide if they can update it on the backend or whether it is a permanent change. So for the moment custom chairs remain broken when slotted to surfaces (this included barstools, dining chairs, and office chairs).