I am aware Jasnah could transform all 10 essences, the point I wanted to make is that people still believed she had a real soulcaster. Since she used it so often in public I would assume she was careful to follow the rules as if she had a real fabrial, or the ardents may have started to query too much.
I'm not sure if Savantism plays a part in it, Kaza is definitely a Savant but she also thinks she is limited to 1 essence due to her soulcaster only having 1 "mode". I am sure she would have noticed if the limitation was more recent.
Now that I think about it Shallan mentions in WoK that most Soulcasters are limited to only a few transformations but there are some greater ones, as Jasnahs is believed to be.
I am not sure what the difference is though as the descriptions are the same, physically speaking.