In most cases of "Work for hire" ownership transfers as soon as the work is done and not based on payment. There's a lot of situations where payment may be late or (more likely) potentially disputed. It's almost impossible to write a contract which clearly states when work is "Done" and a payment due so no one wants to have a dispute over existing work. If this is a long (2 year contract) in theory work from a year ago is not "owned" until the contract is done and "paid in full". This language is shaky in a few places around this point.

I (really) don't know if this is a big deal for due diligence but a hiring party would likely strike this language immediately.