This is quite likely due to a memory leak in 6.5.x and 6.6 that has been addressed in 6.6.3+. This same issue is referenced here as well: Abnormally large jumping num_mutations_to_index in FTS index while it is 100% ready - #17 by abhinav .
I’d strongly urge moving to the latest release in 6.6.x.
To immediately mitigate this issue - I’d try killing the cbft process running on each of your nodes, which should free up the memory that the cbft process is hoarding but has lost reference to.