Hey Andrew,
Great question and I have been doing some digging on this for you.
Here is what I have found (if you look at the bottom FAQ this should help) but it would seem there is no workaround unfortunately.
Keen to hear if anyone has had a similar need. 

Hi ​@Andrew Gabb - this is intended behaviour with the approval workflow, please see our help article that outlines the inspection cannot be marked as incomplete, once approved. Can I please ask what your use case is to require marking the inspection as incomplete, post-approval?

Hi ​@Karen Starr 

The template is for IT equipment supplied for homeworking off the back of DSE assessments. Once the required equipment is selected, it comes through for approval. At this point, the equipment is then issued. 

The end report is kept and used to record equipment issued. DSE assessments are repeated annually. In cases where additional equipment is required, the original form is used. 

If it is intentional that they are unable to be marked as incomplete, it is not a great issue. I will just removed the approval and change it to a signature for sign off