As a practical matter, I think that for at least the next decade, people are at least as likely to want to fill with a composed, multi-BMP-codepoint 'char' (grapheme) as with a non-BMP char. So to me, failure with the latter is no worse than failure with the former.

The underlying problem is that centering k chars within n spaces with fill i is based on one-char per code encodings *and* fixed pitch fonts with one-char per space. That model is not universally applicable, so I do not consider it a bug that functions based on that model are also not universally applicable. Perhaps docs should be clearer about the limitations of many of the string methods in the new context.

A full general solution to the general problem of centering requires a shift to physical units (points or mm) and detailed font information, including kerning. This is beyond the scope of a string method.

So I consider this a feature request for a partial generalization of unclear utility and unclear definition.