Slicer buttons change positions in certain situations

My Week 1 slicer for Feb18 seems to function correctly. However, when I click the week 2 button, then the Mon button, the button for week 2 moves to the top (so that week 1 is now below it). The data and chart are correct for 2-5, but clicking the Tue button (which should be 2-6) doesn't change anything. Clicking the Wed button gives me the correct data for Tue.
I've gone round and round on this, but my expertise at slicers is obviously sorely lacking - can someone tell me how this is screwed up?