Tanimbar
The section I thought may be relevant concerns precipition of absorbed water from Jet A1 as the temp falls. This precipited water will form droplets then sink to the bottom of the tank. SG of Jet A1 0.78-0.80. Ice 0.9-0.95? In the case in point this precipited water will be in the form of ice crystals. Note your comments on microbes. If tanks are not treated with biocide during cleaning then they are another matter, will be present if water present, but are easily identified under the microscope. This must have been checked post incident as they are a well documented contaminant as are fungal spores.
The bottom line is that in the first sentence of ASTM spec for Jet A1, includes the phrase " bright, clear, and FREE from water". Not drops, not ppm, not entrained...none. If this is not what is being delivered on every occasion then there is a quality control issue.