I don’t have June drop on plums. In my experience, plum drops usually cause by lack of pollination ( no pollinating partner) or lack of adequate pollination (rain often during bloom time).
In these scenarios, plum can still form fruitlets, hang on for a while, turn yellow and drop.
One year, my Shiro bloomed profusely but my Satsuma flower buds were killed by late freeze. No cross pollination. Not one single Shiro fruitlets survived to maturity. Last year, we had rain almost continually throughout bloom time, fruit set on J plum was very low.