If you coat a gelatin layer on a film of any kind (or paper), and then allow it to dry, it will contract and hence it will always bend/curl into itself. On 35mm film this is countered by using a fairly heavy/thick substrate and the relatively narrow width of the film strip furthermore limits the issue. Still, also 35mm tends to curl a bit, depending on various factors.
With 120 and sheet film formats, this curling is countered by coating an anti-curl layer on the opposite side of the film. If this anti-curl layer exhibits the exact same drying and contraction behavior as the actual emulsion, the film will always dry perfectly flat, regardless of the conditions. The slights difference between the drying behavior of both layers will result in a curl into either direction. The problem is the worst with 120 film since it's coated unto a really thin substrate, other than sheet film (and 35mm).
It seems that Foma's anti curl layer is a little less effective than e.g. Ilford's; they're just a little less proficient at matching the curl rates/extents of both sides of the film. The result will that Foma's films will curl more, and that this curling will depend to a significant extent on the drying conditions and the final moisture content of the emulsion. A little moisture left in the emulsion will leave it more supple and less curly, so drying film under higher humidity conditions will pose less of a curling problem. Drying film in a very low humidity environment, and drying it for a long time, will result in a very low remaining moisture content and a very curly (and somewhat 'crispy') film.
To what extent manufacturers include plasticizers or even humectants in their emulsions, I don't know. It's perfectly possible (even likely) that they employ plasticizers to reduce curling issues.
