In a system that is operating normally, NEM0 is the primary, and NEM1 acts as a hot spare.
If NEM0 fails, the failover mechanism causes NEM1 to take over. This is expected behavior.
When you replace NEM0, NEM1 continues to operate as the primary, and NEM0 remains the spare. The system is now configured abnormally.
If this is a problem, use the following workaround to make NEM0 the primary and NEM1 the spare.
Workaround
Power down the system.
Remove NEM1 while leaving NEM0 in place.
Boot the system.
The system makes NEM0 the primary. There is not yet a hot spare.
Insert NEM1.
NEM1 becomes the hot spare.