Flow issue: Possibly linked to do the following in parallel?

I have a working flow which I have copied for a new request item which has a slightly different routing with tasks, but my flow seems to break either immediately on saving a copy. If I edit the original flow with the changes I need and test it (without saving), the "new" flow works. I don't want to click save or I'll overwrite my original flow, which we still need. I therefore needed to either create a copy and edit it for the new request item, or start from scratch.

To summarise, the flow:

Trigger:

Catalog item requested.

Actions:

1. Update request item assignment group.

2. Grab catalog variables

## Fulfillment stage entered ##

3. Do the following in parallel:

4-5. --- Create a catalog task

6-7. --- Check if specific variable is set to yes

8. --- --- If the above is yes, create another catalog task.

9. Run a subflow to check for manual tasks and that they're all closed.

## Completed stage entered ##

10. Log to close workflow.

If I:

- Copy the existing working flow and immediately test without any changes, so it's absolutely identical to the original, the flow appears to already be broken (screenshot attached).

- Copy the existing flow and make identical changes to my initial test, the flow appears broken.

- Create a brand new flow, add all the steps in whilst testing each time I make a change,  the flow breaks as soon as I add in the step to do the following in parallel, even if it's identical to the original flow. As soon as I take out the parallel step, the flow works again. 

I identified the flow wasn't right because:

1. When running the flow test, the completed stage appears in a big golden bar instead of grey, and there's only one stage at the top instead of two.

2. When completing the tasks, the request doesn't flow to complete.

I have checked a multitude of things, but specifically:

1. All flows have been created in the Global application scope.

2. All flows are being run as system user. 

3. All flows have the same stages, and I've tried removing and re-adding them on my test flows.

4. Tested running the original flow, new identical copy flow, new modified copy flow, and newly created from scratch flow (with and without the parallel step) all on the same RITM (and others) to make sure it wasn't an issue there. The original flow works fine, as does any copy/new flow without the parallel step, the others all fall down at the same spot.

Where have I gone wrong here?