Orion-Tr Smart DC-DC 12|12-30 isolated connected to busbar on the minus side

“My isolated DC‑DC 12/12‑30 unit is connected to the shared negative busbar instead of directly to the battery terminals. The system includes a 70 Ah AGM starter battery, two parallel‑connected 140 Ah LiFePO₄ house batteries, and a Victron Smart Shunt. Is it bad practice not to connect directly to the battery terminals?”

No, it’s not bad practice.