All asserts pass on all implementations of std::span so far, but is there anything I may be missing that makes this a bug, e.g., UB?

I don't think this would be undefined behavior. After all, std::span is just a class, not part of the core language, so I think this should only be unspecified behavior.

In MSVC's implementation, span1.begin()==span2.begin() can pass the inspection in Debug mode as long as span1.data()==span2.data() && span1.size()==span2.size(),