Unknown yellow substance from mixing ascorbic acid, baking soda, and rubbing alcohol, then boiling
That's sort of the colour that vitamin-C tablets go if they are kept long beyond their expiry date. I'd go with it being oxidation of some sort but chemistry.oregonstate.edu/courses/ch130/old/VITCTEXT.htm indicated that the simple oxidation of ascorbic acid is colourless... so what you've got is a more complex mix of oxidation products.

Instead of boiling off in an open pan, boiling off in a stable atmosphere might work better. You'd probably need some distillation glassware and a source of non-oxidising gas to flush out the oxygen dissolved in the water / keep an oxygen free atmosphere. You could probably get away with any arc welding gas or even use carbonated water as a source of CO2 gas to flush the air out of your equipment. Or of course you could go the whole chemistry-department route and boil the water off in a low pressure system without adding much heat.