Why did SNMP polling for the router fail after adding an interface and rebooting?

The SNMP interface index (ifIndex) is a unique identifier assigned to each network interface on a device. On some older or simpler network devices, especially after configuration changes like adding or removing interfaces, the ifIndex values can be reassigned upon reboot. If the monitoring system (SNMP manager) was polling based on a previously assigned ifIndex, and that index now corresponds to a different interface or no longer exists, polling for the intended interface will fail. The SNMP server traps being disabled for link state or interface index are irrelevant to polling failures, as traps are unsolicited notifications, not responses to polls. An incorrectly configured SNMP community would prevent any SNMP communication, not just polling for a specific interface after a reboot.