I'm no expert, but I suspect it's just a throwback.  One of my ADFs has a freckled belly as well (not the norm for bred frogs), and it doesn't appear to have hindered him.  His freckles appeared later, in adulthood, not when he was young.  They kind of crept in from the edges (his sides), which were of course already freckled.  He's had a freckled belly for years and he's still kicking!  Your frogs aren't the same species so I hesitate to count too heavily on ADF comparisons here, but your frog's freckles look a lot like the ones on top, only sparser -- they don't look odd or unusual like they're some kind of growth or something.  If there's nothing else alarming going on and nobody else chimes in to say pigmentation changes in adolescence are a symptom of some known disease, I wouldn't worry about it.