All of my trees have been exposed to FMV. This one has had some years when it showed, and other years when it didn't show. In my opinion, taking care to ensure good nutrition/culture, including attention to micronutrient needs (aka "trace minerals"), goes a long way toward helping any fig tree overcome the effects of those viruses. It can even make it appear as though there are no symptoms from fmv at all. Conversely, letting a tree get rootbound, or overwatering, or other maltreatments, all of those things seem to exacerbate the deleterious effects of the viruses. So in the years when I can keep up with potting needs and treat them well, the trees show almost no effect. Meanwhile, some of the trees almost never show any symptoms at all. There was a guy on here, ascpete, who posted some really interesting results along these lines (I think it was sometime between 2012 and 2014, if you're interested to search). Anyway, all of my trees have been exposed.
Mike
p.s. That's not to say that's what's going on with your tree (I wouldn't want to imply that!)... just sharing my experience. I suspect that some specimens can just "get it bad" (just as some others seem "naturally resistant" and never show it). Or maybe they're weak in other inherent ways (e.g. a poor immune system, if trees have such things), and maybe that's what's up with yours. I do see differences in this regard among different individuals of the same cultivar though.