Comment 4 for bug 1887228

The major change between 2.7 and 2.8 is that in 2.7, hook changes are applied _individually_ in a best-effort approach. This means that if any of the change operations fails, juju will still attempt to apply the rest and return back the first-seen error. On the other hand, in 2.8, all hook changes (pod-spec included) get applied in a single transaction and all changes get rolled back in case on an error.

AFAICT from https://github.com/juju/juju/blob/2.7/worker/uniter/runner/context/context.go#L878-L883, 2.7 defers setting the podspec until the hook flush time so the issue with 2.8 might be caused by the combination of changes being applied together.