It's fundamentally very similar to the sorts of issues you end up with if you compress then encrypt. If the attacker can make some educated guesses about the plaintext prior to the compression, the compression ratio can be a very powerful tool in their arsenal.
https://www.cs.jhu.edu/~cwright/oakland08.pdf
https://www.cs.jhu.edu/~cwright/voip-vbr.pdf
It's fundamentally very similar to the sorts of issues you end up with if you compress then encrypt. If the attacker can make some educated guesses about the plaintext prior to the compression, the compression ratio can be a very powerful tool in their arsenal.