Why did MongoDB 2.6 use disk space grow indefinitely when using TTL ?

Question

MongoDB

It was MongoDB 2.4.9
There is a large collection in which TTL is used for documents (we store them for 24 hours) - docs.mongodb.org/manual/tutorial/expire-data
Everything is great. On average, the same number of new documents came in per day and left by TTL. The files on the disk with the collection occupied 80 gigs and IMPORTANT!!! their size did not grow with time!
MongoDB was updated to version 2.6.3 on July 15.
And since that day (Zabix monitors) the used disk space has been constantly growing! It's good that when the threshold value is reached, the SMS came on time. Managed to respond.
Actually the question is: did something change in 2.6.3? No one has ever heard of such behavior. And is it possible to return the behavior as it was in 2.4.9?
PS Now we get out like this. We have two equivalent servers in the replica set. On secondary, we delete the entire database. He pulls it out from primary. We change roles and do the same on the second node.

Here is a chart from Zabix by place used