As per the discussion over in issue #10181, I've changed my position on this issue.

Since the PEP isn't explicit on the exact semantics here, I think we should be guided by the memoryview behaviour and make it official that bf_releasebuffer implementations shouldn't trust the contents of the passed in Py_buffer struct. Instead, if that information is important, the exporter should create an internal data structure that preserves the necessary detail and merely use the Py_buffer address as an identifier to locate the internal copy.