Thumbor request URL

Broken:
Original: https://s3-us-west-2.amazonaws.com/app.local.getstencil.com/upld6878b91f
Resized: https://d32q3izcmhqbti.cloudfront.net/174x0/upld6878b91f

Working:
Original: https://s3-us-west-2.amazonaws.com/app.local.getstencil.com/upld65b35218
Resized: https://d32q3izcmhqbti.cloudfront.net/174x0/upld65b35218

Expected behaviour

Any transparent PNGs that are resized should have their backgrounds kept as transparent.

Actual behaviour

Some are having black pixels inserted in place of transparent pixels.

Operating system

OSx / Chrome

Your thumbor.conf

Using through an AWS template, so don't have the config file handy.