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First guess would be that your SNMP security settings are not allowing the third poller. ICMP will still work, so it won't show the device as down, but with SNMP unavailable, all interfaces and statistics stop.
I would move it to the bad poller and run the snmp credential test.
The only caveat would be if NPM always runs the test from the primary NPM server and not from the poller you've assigned. If that's the case - it would be time to submit a bug request.
Turned out to be the Access List on these devices allowed polling from the first two pollers but not the 3rd. Network is updating their rules for us.