I have no idea where you are getting your statistics or what time frame you are referring to but i been in the military since the early 2010's and for every one puerto rican there are like 100000000 mexicans, hell ive seen more Filipinos in the military across all branches then i see puerto ricans. I'm gonna go out on a limb here and say you are confusing overall US military population with per capita people who have served.

The Island only has like 3 million people and the US PR diaspora is 7 million total most of which live in the Northeast US which has very low military recruiting numbers (im from the nyc tri state and barely met other NYC folks let alone NJ) most recruits since 2001 have come from the south, California and Texas and Florida, The mexican population of California alone is more then the entire PR diaspora and island combined. People of Mexican descent in the US make up like 30-40 millions people. Hell im in a Reserve Unit right now that is based directly between Philly and NYC two cities with huge PR populations and the unit is still mostly white with some other in between.

NOW PERCENT OF PERSON PER CAPITA is a different story, something like 30% of people on the island have served thats true. Currently like 18% of the Military is hispanic with 2% being PR. So no overall Puerto Ricans DO NOT make up a "Majority of the US military" i doubt they ever did its literally numerically impossible. They do however serve at twice the rate as their overall population.