Do you protect it from your winters? Even in my Z6 it is a bit fragile, and I’ve lost trees to much less than test winter cold. I lost a tree in Greenwich this season that probably faced nothing lower than -10. Santa Rosa is also pretty tender, although the general nursery industry has been led to believe it is a hardy one.
I lost my first Satsuma in my own orchard a few years ago, but had already planted another at another part of my property- they were both planted at sites with good drainage, both for cold air and soil, nearly identical soil and elevation. This is always a bit of a mystery about winter and spring cambium kill- what lives and dies is quite unpredictable although certain varieties and species are certainly more susceptible to injury and death.
I recommend that all people in Z6 or colder start their J. plums on Shiro and graft what they want to that. I haven’t lost more fragile ones grafted to Shiro yet- but this is a fairly new approach. At least the mother tree is likely to survive. It also has a more cooperative growth habit than the rank growing Sat. Less work to develop a nice shaped tree- at least on myro.