the source of your assets is generally speaking where your tax home was in the year you obtained them.
Example:
if you bought an asset while living in the USA in January 2019, move to PR in march 2019 and fulfill year of the move requirements to be a BFR-PR for the entire year, then the source of the assets is actually PR
there are special rules, like the 10-year securities rule, that can overwrite this general assumtion