The lawyers in the UK have done something odd. They’ve decided that if someone commissions a bit of work from someone else, the copyright of the work remains with the author unless the contract explicitly says otherwise. The details are here.
It’s hard to imagine any circumstance in which X would pay Y to create an original work and not intend to acquire ownership. So this is just a typical lawyer’s trick, designed to increase the income of lawyers by obscuring what was plain to everyone and forcing everyone to state in writing what everyone presumed already, on pain of enriching more lawyers. I can’t stomach such things.
I’ve tended to state this explicitly in all the work I have commissioned, being paranoid, but I’ve written to all my people explicitly again. What the US position is I do not know.
As a rule I only take ownership so that I am free to give it away, of course.