I came here to ask the same question! Im sure it does fumarate salts.
There appears to be almost no data on the solubility of NMT fumarate. Would NMT fumarate force out of solution DMT fumarate or vice versa?
It appears that there is something from above d-limo salting FASW approach that precipitates out at room temperature that looks like much more than the amount of added fumaric acid. This hasnt been weighed so of course it is possible the glass and water are making it look much more than it is and that it is simply fumaric acid.
What has been observed is that you can remove much excess fumaric acid by freeze-thawing the FASW, comparing the structure of the crystals with the non-salted fumaric acid solutions' precipitates leads to the suggestion that fumaric acid is practically insoluble in dmt/nmt fumarate solution at below freezing temperatures.
Although not ground breaking it is good to know as less fumaric acid means less sodium carbonate in the past and pull method and therefore less solvent needed to separate the two and the excess fumaric acid can be recycled.