Its the seating stem that's loose, and backing out. I use the Hornady lock rings on all my dies.
I didn't realize the microjust could be used with pistol seating dies.
Are these the ones you use?
https://www.midwayusa.com/product/1011035763?pid=255580
https://www.amazon.com/Hornady-04409...s%2C685&sr=8-1