[NTG-context] Why do size changes affect my math font?

Consider the following simple document, with is a minimalist version of the slides that I'm currently working on: \starttext The variables are $u$ and $p$. \stoptext This works fine; the text is in roman and the math in italics, just as I desire. However, I need a larger font for my slides, and thus I'm also using a font size command, like so: \starttext \tfc The variables are $u$ and $p$. \stoptext This causes the math to be in roman instead of italic. (It's not in "text font" per se; an \it after the \tfc will italicise the text but not the math, creating a quite odd effect.) Is there a reason for this? What's the cleanest way to fix it for all the various size commands? Thanks, - Brooks