The difference in ARP entries for both the nodes is normal. Reason being, when we enable IST, only the FDB entries get synced and not the ARP. Hence, we see same number of FDB entries on both cores. Example below:
Core1: 7591 out of 7591 entries in all fdb(s) displayed.
Core2: 7591 out of 7591 entries in all fdb(s) displayed.
The reason why we are seeing the difference in ARP entries is because the ARP sync takes place between IST/vIST peers only when the IST first comes up. After that, there is never again an ARP sync. Each node learns ARP independently of each other. So traffic patterns and end device types play a huge roll in how ARP tables get populated.