Hi @Stella,
Thanks for contacting Microsoft Q&A platform.
I understand you have some queries around why your low-priority VM is being preempted. To start with, low priority VMs are effectively discounted VMs that are allocated from the unused capacity of Azure VMs in a particular region, to anyone who requests for it. They are heavily discounted and available to run workloads running in Batch.
you can follow below link for more insights.
Hope this helps you.