The other problem with common law is that there's no categorical statement. It's like science - you have to discover the law by inference from observations, and sometimes thought-experiments.
The simple theoretical model is that a peerage dignity is an entity that is created and descends through heirs and becomes extinct. But it's a flawed model - it doesn't totally reflect reality.
The reality is that you can't always tell when a new title is being created. And the reason you can't tell is that it makes no difference.
In jure uxoris cases, there are some tests they can apply.
1 - if the title passed to a child of the marriage, on whose death did it descend?
2 - who else could inherit? Could the husband's title have passed to a child of a different marriage?
3 - what was the precedence? New titles normally go to the back of the pecking order.
But none of these tests is absolute. A new title was sometimes given the precedence of an old one. Sometimes by mistake, but the law has always been very reluctant to recognize mistakes.
Sometimes a son took over from his living mother. And sometimes a father clung on to what should have been his son's.
Sometimes a new title came with a special limitation. But then again, if you limit the husband's title to heirs of the marriage, you could as easily say it was for life only.