all right support finally got back to me, apparently the expected behavior is that the previews expire after 14 days, for data retention policy reasons... this is documented deep in the privacy section of the slack app details page
interesting, they don't seem to agree... yes, I definitely see some purple in the color, but it's so little as to discount it... like when green has a bit of blue in it, it's still green
I have a rather silly question: is the default color for line graphs blue or purple? It looks unambiguously blue to me, but everyone else on my team insists it's purple, and I want to know what the original Mixpanel intention was