Excaberating this is the tendency of contractor to be less than enthusiastic regarding documentation (why document when it just makes you easier to be replaced/not be renewed?) and the low prioritization of documentation in daily operations which makes swapping out highly technical human resources a significant challenge.
If the contractor isn't writing documentation as part of their day to day work, They're not the kind of contractor I want anywhere near business critical systems. That demonstrates a worrying level of professionalism. IMO, It's akin to a sysadmin placing logic bombs in case they get fired. On the other hand if their manager isn't allocating time, that's a needless risk to the business. Either way, it's an HR issue.
A lesson I learnt long ago is that "Nobody is irreplaceable". Long ago that mindset was prevalent in many developers, thankfully things have improved in the last few years. Most of the development shops these days, run on the "Build it modular enough that if it ain't documented, then it's replaced/reworked by someone else at the first opportunity" principal.