Hi Nick,
It is sometimes confusing. Please note that while the waveform looks like half step, it is not really. The DRV8711 sets current at 71% and 100% for half step. This appears to be 50% and 100%. Also, the max current appears to be twice the current of the other scope captures.
A quick experiment to see the effect of slow decay is to lower the voltage to 12V, assuming you are running at a higher voltage.
An example of loss of current regulation can be seen in the blog ( HERE )