I have the same issue but have potentally noticed the problem I just dont know how to fix it.
I have followed the instructions and deleted the Spotlight-V100 in the time machine sparsebundle which all works fine I do a back up and again no problem. 🙂
It has been working fine backing up ejecting all mundane however when it feels like it 2 hidden files are created on the Backups.backupdb folders which are .spotlight_repair and .spotlight_temp the folder appear to stop the drive from ejecting
this is a copy of my log that is a usb drive connected from the Airport
Starting standard backup
Attempting to mount network destination URL: afp://Steven@Base%20Station%20aecf91._afpovertcp._tcp.local/TIME%20WARP%20500
Mounted network destination at mountpoint: /Volumes/TIME WARP 500 using URL: afp://Steven@Base%20Station%20aecf91._afpovertcp._tcp.local/TIME%20WARP%20500
QUICKCHECK ONLY; FILESYSTEM CLEAN
Disk image /Volumes/TIME WARP 500/My MacBook.sparsebundle mounted at: /Volumes/Time Machine Backups
Backing up to: /Volumes/Time Machine Backups/Backups.backupdb
100.0 MB required (including padding), 148.53 GB available
Waiting for index to be ready (100)
Copied 1439 files (9.7 MB) from volume MacBook HD.
100.0 MB required (including padding), 148.53 GB available
Copied 1351 files (1.1 MB) from volume MacBook HD.
Starting post-backup thinning
No post-back up thinning needed: no expired backups exist
Backup completed successfully.
Waiting for Spotlight to finish indexing /Volumes/Time Machine Backups/Backups.backupdb
Waiting for Spotlight to finish indexing /Volumes/Time Machine Backups/Backups.backupdb
Waiting for Spotlight to finish indexing /Volumes/Time Machine Backups/Backups.backupdb
Waiting for Spotlight to finish indexing /Volumes/Time Machine Backups/Backups.backupdb
Waiting for Spotlight to finish indexing /Volumes/Time Machine Backups/Backups.backupdb
Waiting for Spotlight to finish indexing /Volumes/Time Machine Backups/Backups.backupdb