Not sunburn but a tan, due to increased light levels. There's nothing wrong with it.

I believe it is burned. Has happened to quite of few of mine lately, with plants I thought were tougher (in my SOS program-sink or swim). A "tan" would be much prettier, IMHO. =) Mine, with a tan:

{{gwi:493636}} {{gwi:661313}}

Yesd, burned to me too.
This happens especially with S. adolphi (and friends) overhere. And that really is a puzzle to me. Our sunpower is much lower than eg in Southern-Europe or California and still every time they burn.
Harry

Burned, but only slightly. Since they shed the leaves, no permanent damage done. This shows the difference between good light indoors and almost shade outside. Outside light is more intense by several magnitudes, but better for most plants.