Pearl have been a subject of many fan theories like who is her original master but she always keep everything a secret! I don't know if Garnet knew who was Pearl's Original Master but maybe she knew or not.
Since Eyeball outright tell Steven that Rose KILLED Pink Diamond and even Garnet confirmed about it. I felt the antagonist should be the one that tell Steven about Pearl's secret! That she was once owned by White Diamond because hey! Let shake up either Season 4 or Season 5!