Mexico could declare escaped slaves were free all we wanted, that doesn't mean slavers would care, so I wonder, was there ever some sort of legal battle concerning this law?
I wonder if any point something like this happened:
A slave escapes into Mexico but is then found by USA authorities and brought back to the USA by force. However this person explains that since they entered Mexican territory before being captured again they are now free and a Mexican citizen, so capturing him is now illegal. Such a situation would have created the question of how the laws from other countries concerning slavery would affect slavery in the USA
/u/Georgy_K_Zhukov and /u/drylaw have previously answered During the time of slavery in the United States, why did the slaves flee to the North instead of going more South to Mexico where slavery is already abolished?
See also Why did African slavery and plantation agriculture not dominate colonial Mexico the way that it ruled nearby regions of Cuba, Brazil, and the American South? feat. /u/anthropology_nerd too. Anthropology_nerd has written a lot about colonists enslaving Indians.