I also noticed that Amethyst often seems to look like Rose lately. In the Steven Bomb and three gems and a baby, and maybe one more time? I want to look again to see. It might not mean anything, could just be a coincidence. I've only noticed this in the most recent episodes, anyway.
I would start by saying that as Quartz gems, Amethyst and Rose would have similar body types and features already. But I think that the ask is referring specifically to their faces and expressions.
So I looked through a few photos of Amethyst in more recent episodes, particularly Three Gems and a Baby, mentioned in the ask.
Amethyst looks most similar to Rose when she’s smiling widely and her eyes squint a little in the process.
Because her lower lids move upward and give her eyes the shape of semicircles. This is very similar to Rose’s eye shape. At this point, they already have the same full flips and small noses. They both have round faces and big hair that frames their face in a similar manner.
I think what’s always been a differentiating factor was the eyes then. In other recent episodes, such as in That Will Be All of Steven Bomb 5, a smiling Amethyst doesn’t really call back to Rose unless you deliberately make the association (in which case all the other mitigating factors in their similar gem type and appearance could sway your decision).
I’d say the implications of this aren’t too wild. Because of the already present similarities, and the limits of the aesthetic used in drawing characters in the show (in addition to the changing story-boarders), it wouldn’t signify too much about Amethyst’s character. I think a more definitive similarity would be telling though!