Disk drive spin-up time differs for disk shelves with four power supplies and two power supplies. For a disk shelf with four power supplies, all disk drives spin-up at the same time. For a disk shelf with two power supplies, each of the four columns of disk drives spin-up at 12 second intervals.
It seems that adding 2 power supplies should solve the issue.
Otherwise, I've been told that removing the SAS cables before powering on the shelf is recommended.
Well, NetApp ships SATA shelves with 2 PSUs and official statement is, 2 PSUs are enough for SATA. It is true in the sense, shelf with SATA drives will function with single PSU. But as we have seen it could cause issues.
It is really independent of Metro Cluster (although having two pools makes it worse). Consider short power outage. FAS and shelves will power on when it is over and this may trigger excessive reconstructions that could be avoided. And even with 4 PSUs there is no guarantee that all 4 of them will be powered if outage happens.
But returning to the question of spare selection - it looks like a bug. According to KB I mentioned
Data ONTAP will search for suitable spares in the opposite pool only if the aggregate is mirror-degraded or is resyncing, with the plex containing the failed disk serving as the source of the resync.
according to your description that was not the case. You have "failed" disks in target part of resync in mirrored aggregate. It should not use spares from different pool then. On your place I would open separate case pointing to this KB article and asking to clarify.