Like a lot of problems, there are several possibilities. Unless it a simple excess of salt or cream of tartar, I'll take the "salty" taste to be not necessarily sodium chloride and maybe an different but bad taste.
It is possible for any acid to react with stainless steel to leach metals, maybe nickle. Depends on the specific formulation of the steel, and I consider it the least likely, but it can happen
Oxidized sugar can also taste metallic. And powdered sugar is the most sensitive, because the fine grind exposes far more surface to the air than table sugar. I consider this one a high probability. We all tend to keep powdered sugar a long time.
And another, less likely, is a reaction with the anti-clumping agent added to that particular sugar. None of them are truly tasteless, but more important, unless you know what they are, you can't predict reactions. Commercial powdered sugars don't have them. They're used up so fast that they don't need them, and for the same reason, commercial sugars don't get a chance to oxidize.
Finally, one that may explain very sporadic bad tastes in citrus desserts. Oleo-Saccharum is a substance made by proper titration of volatile citrus oils with sugar. It is a valuable product in wine finishing and in punches and other citrus drinks where it imparts a highly fragrant quality. But there are differences in the volatile oils that come off citrus over time. The small quantity of first oils can produce an unpleasant tasting oleo-saccharum, while the bulk of the oils, coming off after, produce the good product. In the inadvertent formation of o-s in carrying out a recipe that involved citrus and sugar, simple bad luck would be involved in the formation of the bad taste. Is the off taste from the meringue or from the filling?
A real problem is that there are enough reports of very sporadic off tastes in meringues that it's hard to reproduce a problem. You might do it again exactly the same way and have it come out fine.
"Kitchen duty is awarded only to those of manifest excellence..." - The Master, Dogen