On Cloud, the Sub tab in [3] stays empty for most Epics, even
when they clearly have children. On Stories the subtasks show up
as expected.

The tab renders from Issue.Subtasks, and on Cloud that field
only contains the subtasks of a Story, not the children sitting
under an Epic via the parent link.

Proposal: on Cloud, fetch the children with parent = <KEY> and
use that as the Sub tab content. DC needs a different query and I
don't have an instance to test against, so it keeps the current
behavior.

I have this implemented and can open a PR.