Why is this Lilly pilly dying/dead?

Lilly Pilly Straight & Narrow. Was planted about 3 months ago and growing well. All of a sudden started to dry up. The Lilly pillys surround it was completely fine (though they were planted 2 years ago).

I have noticed grubs in the soil but assumed they’re beneficial? The mulch is eucalyptus mulch. Has Osmocote slow release fertiliser underneath drip line irrigation.