The reason for the Mexican arm bend is that the Spanish king did not condemn the injuries that the Spanish conquerors – the conquistadors – inflicted on the region when they ravaged it in the 16th century.
The outgoing President Andrés Manuel López Obrador wrote a letter to King Felipe in 2019, in which he asked the Spanish king to "publicly and officially" acknowledge the "injuries" that the indigenous peoples suffered during the Spanish conquests of 1519-1521.
The king did not respond.
The diplomatic complications between Spain and Mexico are now worsening even further, as the invited Spanish Prime Minister, socialist Pedro Sánchez, is also boycotting the ceremony, citing that the presence of King Felipe is "unacceptable".
"The Spanish government has decided not to participate, in any way, in the ceremony", the Spanish Foreign Ministry wrote earlier this week.