Incremental Merge and BCT file

In one of our setups, we are using Rubrik and utilising the Incremental Merge feature of backups.

The setup has clearly taken a Level 0 equivalent on the first run and after a subsequent 3 incremental copies it has started to merge the incremental backups with the image copies.

However, a Block Change Tracking file is being used to 'speed up' the incremental backups and I wondered, how can an Incremental Merge forever be consistent in terms of timing if the BCT file only has 8 slots (default) per file.

I understand that if Incremental backups are taken and the BCT file is left as is, then after 8 incremental backups the slots in the BCT file will be full and RMAN will go back to scanning every block in the file.