I have received 2 alerts about a dtrace.core file since I upgraded to the latest TrueNAS (version 12.0-U2.1). These are new for me, never saw them, so I guess something changed after the upgrade. I don't see any impact on the system though, it appears to run normally.
The instructions in the alert were to create a ticket here.
I have attached the dtrace.core file (I zipped it, hope that's OK) and also the debug-output from the UI, since I saw that was often requested in similar reports.