Less than 50 years after the Vietnam War, and 70 years after the China Burma India theater of WW2, the memory of molds and fungus attack on equipment has seemly been forgotten.
Equipment exposed to hot, humid, semi tropical or tropical environments quickly gets inhabited by hungry molds and fungi, all looking for munchies. The WW2 generation found to their horror, electrical insulation, rubbers, early plastics, etc, all being eaten by molds and funguses. And sometimes, themselves. I knew a WW2 Veteran (Dr Ben) who had been a Weatherman. I pooh-poohed him as a REMF until he explained, prior to Allied invasions, he and Metrological group were dropped behind enemy lines on the China border and the Philippines to tell the High Command the weather prior to the invasion! Metrological units would set up weather stations, at high elevation locations, and at the bottom of these hills, were Chinese bandit gangs on the way up for a visit. Dr Ben described mountain fire fights with Chinese bandit gang looking for goodies, and quite willing to kill round eyes to take their possessions! Dr Ben survived, and so did a permanent skin fungal infection. The skin had been peeling off his hands for more than a half century, and I don’t think it ever stopped.
I am unaware of specific mold and fungus testing prior to WW2, but after WW2, molds and fungi are recognized as threats to equipment deployed world wide. And so, you can find things like this:
Fungus Testing Lab
Standard Practice for Determining Resistance of Synthetic Polymeric Materials to Fungi
As to what mold or fungus is eating your leather, heck if I know. Some have estimated the number of fungi species as 2.2 to 3.8 million, with only hundreds of thousands identified. There are a lot of fungus among us. (My Dad just to joke about that) The reason one sling is being eaten and not the other is probably because of the tanning and dyeing chemicals in one, is more tasty to the little buggers! Could be the hide, maybe your fungi have a preference to Uruguayan cattle than Brazilian.
This will not get better, outside of dry environments. Fungi love hot moist, so does mold. I don’t know a cure. I am sure that anything that will kill fungi, will also damage your leather.