I've noticed that the ContrastChecker does not work well with the Cover block's Overlay and Text colors. Overall its very inconsistent, but I have noticed a few things in particular:

A. The notice shouldn't be fired in the default state (no text and overlay colors):

B. The ContrastChecker tends to five a false positive for accessible colors. i.e showing the notice when the color does have appropriate contrast. The checker should use to the Overlay fallback color (perhaps that should be set in the attribute, instead of CSS directly):

C. Changing the Overlay color never fires the ContrastChecker.