That doesn't mean it was formatted, you need to manually format every disk after it's added to the array, but that's not the case here, the disk was formatted before, though based on the transid it was barelly used:
Jun 9 00:40:52 BIGASSNAS kernel: BTRFS: device fsid 0ce0630a-bf7e-4ce4-84f1-f598405bcb82 devid 1 transid 7 /dev/sdb1 scanned by udevd (1282)
Note 'transid 7', this is a very new and unused filesystem, compare for example with your other ones:
Jun 9 00:40:52 BIGASSNAS kernel: BTRFS: device fsid d9e9f710-02c9-41cb-bd9b-cfc93345fbc1 devid 1 transid 98843 /dev/sdc1 scanned by udevd (1289)
Jun 9 00:40:52 BIGASSNAS kernel: BTRFS: device fsid 33232feb-7c0b-4e3c-bbef-a0925aae03c4 devid 1 transid 1334443 /dev/sdg1 scanned by udevd (1289)
Jun 9 00:40:52 BIGASSNAS kernel: BTRFS: device fsid 41637979-6e3a-4dfc-a801-edcd1a8032b7 devid 1 transid 113884 /dev/sde1 scanned by udevd (1289)
Jun 9 00:40:52 BIGASSNAS kernel: BTRFS: device fsid 812a0302-1349-4d0e-9703-5291c65db4d8 devid 1 transid 19303823 /dev/sdd1 scanned by udevd (1289)
So if you just re-format pretty sure there wouldn't be much there, maybe nothing, but if you want to try and recover or confirm first there are some recovery options here.