It's not supposed to but it should...
uPlot currently uses a separate 20-color palette and sparkline is drawn by uPlot. Hence it only knows about the 20 colors (and #000). I want to redo that 20-color palette using the MD colors, it'll mean the colors are a bit less "maximally distinguishable" but the current 2-palettes situation is no good. Since sparkline doesn't need umpteen distinguishable colors I should perhaps see whether I can easily switch it to the MD palette in the meantime...
Background on all this: Colors and themes - FlexDash
Thanks you for flagging it!
