After applying redactions other information is removed

This is an interesting discussion. Having thought about it, it's entirely correct that the info is removed on other pages, but it would be nice to get a warning about it. The important thing is to remember the #1 job is redaction - removal of sensitive info - rather than convenient and selective text deletion.  

I should first observe that the job of form XObjects is to allow shared content to be duplicated. A common use is to allow the same background on each page, without having to make the file huge with endless copies of the graphics. It is normal to have these made automatically, for example the PDF Optimizer will do this. The form Xobject is about appearance, and isn't a convienient collection of organized graphics.

Scenario 1: a graphic contains personal info (shall we say, the address of a person hiding from an abusive partner, just for an example). The graphic is used on page 1 and 3. The address on page 1 is covered up. The address on page 3, being part of the same graphic, is also removed. This seems correct. You can argue that it's the responsibility of the redactor to see and select the other address, though.

Scenario 2: a page is split into 9 smaller pages by making the page into a form XObject, duplicating it 9 times, and cropping each one. (This is a real thing).  You redact an address on page 1. But you don't realise the address exists on 8 other pages, cropped out, easily accessed by the attacker. Happily, the XObject takes care of this, and it disappears in the invisible parts too.

Anyway, I feel that this is very much the right thing for Acrobat to do, and using techniques to split out/separate duplicate form XObjects is actually very dangerous in a redaction context. 

Have you tried flattening before redaction?  Redaction will remove any object outside the page content (field or annotation)  if it touches on a redaction area.