As you say,there's not a great deal you can do Stephen,JMs being a comparitively soft bark plant(and I believe named so somewhere)will often get die back on the youngest growth.Some more than others,the thin twigs are easily damaged by the elements and infections.When large portions of die back occur it's usually a sign of something else,more often an infection.All I can suggest is use a copper spray at fall time to help keep out infections and shelter from the worst of the elements,though not always possible.Also don't fertilise late in the year encouraging new growth that won't harden off before dormancy.
I now spray copper every year as we rarely have freezing temps anymore in the south to slow down the spread of infections during dormancy....my plants suffer greatly.

Thanks very much Houzi for that helpful Information

Is the Copper sprayed over the entire Tree during Fall Time ?

What brand are you using please.

Thank you

Stephen

Yes,spray the whole tree and it wouldn't hurt to repeat it again during the dormant season.Actually it was Bordeaux mix(a mixture of copper&lime used in france for ages) I was using until recently.It has now been withdrawn from sale by Eu regs. but a 'copper mixture' is still being sold as a replacement.I think you'll have no trouble finding it in any garden centre...Vitax is one of the brands available.