This bug is nearly identical to #12121, but affects AnchoredText instead of TextArea.

The documentation for AnchoredText strongly suggests that the keyword argument prop should be a FontProperties object. However, inspecting the source code suggests that prop should instead be a dictionary of keyword arguments that will be passed to the TextArea object inside of AnchoredText.

I propose rewording the documentation to make it clear that prop should be a dictionary of keyword arguments to construct the TextArea instance.

Perhaps it would be worthwhile to look over the documentation of other classes in offsetbox for similar issues, since this is the second time I've come across the same issue.