Triggering runahead tasks causes workflow to finish early

[MetRonnie]

To reproduce:

[scheduler]
    allow implicit tasks = True
[scheduling]
    cycling mode = integer
    runahead limit = P3
    [[graph]]
        P1 = foo

When starting this workflow the first 3 cycles spawn and 4/foo is in the runahead state.

If you quickly trigger 4/foo while it is in runhead (before the other tasks have finished running), it will start running and when all tasks finish, the workflow shuts down (permanently finishes) even though there is no final cycle point.

Pull requests welcome!
This is an Open Source project - please consider contributing a bug fix
yourself (please read CONTRIBUTING.md before starting any work though).

[oliver-sanders]

I think I can see what's going on here.

[hjoliver]

This is related to how we auto-spawn next-instances of parentless tasks on release from runahead.