Redactions are distorted - why?

When I open a file after redactions have already been made and saved, the redacted areas (blacked out boxes) are distorted all over the page. This does not happen with all my pdf files so not sure if there is a certain file type that is not allowing the redactions to be placed correctly. The file looks fine while I'm applying the redactions - it's just after I save and try to re-open it. I've attached a snap of what it looks like. Is anyone else having this issue?

My solution in the interim has been printing out the document, scanning it and then manually redacting the content.