If I were you, I'd contact Dell Support about this issue, since from a quick search it's evident that Dell has monitors that also contain cameras for Hello Face, so it's likely that there's some type of issue causing an interaction between Windows Hello and what a Dell system believes is the correct configuration and monitor/camera to use.
Since those issues would be specific to Dell systems, their external monitors and in fact your own specific device and possibly configuration as well, it's something that Dell Support should know much better than a random person here. though you can leave your post here on the off chance someone else has had the same issue in the past and sees it.
Rob