Even if the pouch diverted it is still exposed to contamination, just not as much.
I had a leak post op (presumably) that sealed up on its own, but a huge abscess developed within a month. There are numerous factors that go into whether these things resolve or simmer and fester. It could be inadequate circulation, ongoing IBD inflammation, undetected infections, reactions to suture/staple materials, etc.
The main thing is that the longer it goes on, the more difficult it is to deal with.
Jan