During a case spin-off, "pyWorkPage" corresponds to the parent case and is the owner of the Thread that creates the child subcases.
If a child case creates a Top-level page in an Activity prior to the child case's first assignment, you would expect that Top-level page to belong to, be created within, the parent case's Thread.
There is no child case Thread until you open the child case separately.
The Top-level page that is created prior to first assignment is there for use by the child case before it reaches the first assignment, but you would not expect the Top-level page to be there when the child case is opened.
If you do not want child case-created Top-level page to remain in the parent case's Thread remove it after the spin-off has completed.
Ideally the case child that created the Top-level page should remove it.