I simply mentioned it as having been discussed here before. Personally, I would lean more toward a crystallization artifact perhaps from decomposition of the PVA. As for fungi actually growing in iodine, I agree that iodine is/was widely used as an antiseptic, particularly its complex with PVA. However, a fungus growing between layers would not be exposed to it.
Bottom line: There is no definitive answer and those bothered by it, so far as I can tell, have not investigated it further. Disassembly and microscopic examination to identify which layer is affected would seem to be a first step. If it is in the polarizer, there are simple things that could be done to help pin its cause down.