A child of recognized foreign diplomat, if born in US soil, however, is not a US citizen at birth. And if they try to claim that later in life, it will be denied.

This has always been like that. All jus soli countries do it.

Correct. Children of diplomats born on U.S. soil aren’t covered by the 14th amendment, even though they are subject to U.S. law and can be sued in U.S. courts over many things notwithstanding diplomatic immunity.