[...] I noticed that the silk screen was starting to yellowHas anyone else noticed this before?
Yes, and that is the sign that the board was heated up too much. You will notice that professionally assembled boards don't have this yellowing.
The likely culprit is radiant heat. Even though the air temperature may be much lower, the surface of the board has heated up higher than that, since it locally absorbed the light from the heaters, and FR4 thermal resistivity is rather high compared to even the lousiest of metals.
Whether overheating the board itself means much for the capacitors - who knows. The caps could have been much hotter than the board, or cooler, depending on how the oven is set up.
