Julian...this may not be acid wash specific. It is rather stange that it should appear so late in your water wash cycles but I have seen something similar. At the end of the production process, after the 7%water wash, I often see this type of anomoly in the bio that I drain from my pipe work. There is often about 3-4lts of bio in the pipes and I never introduce it to the wash tank before it has settled. there is normally 3 diffinative layers...glyc, what you describe in your OP and the bio above.
I plan to produce next weekend and I'll endeavour to take some photo's and also do some soap tests following each wash.
Nige