::slotted pseudo elements allow tree-abiding pseudo elements to follow according to [1], but ::part is not specified as a tree-abiding pseudo in [2] or [3]. Still, it would make sense to allow ::part after ::slotted. Also, ::placeholder, which essentially is a ::part is defined as tree-abiding in [2].

[1] https://drafts.csswg.org/css-scoping-1/#slotted-pseudo
[2] https://drafts.csswg.org/css-pseudo-4/#treelike
[3] https://drafts.csswg.org/css-shadow-parts-1/#part