Verbatim self-plagiarism in introductory sections I'm reviewing an article where the authors are kind of experts of a particular subject and published several similar studies in the field, some even in the same journal.

This apparently leads them to reuse part of their own content between articles, sometimes self-citing sometimes not.

E.g. there are a couple of paragraphs in the Introduction that are a copied almost verbatim from a study from the same authors (which they don't cite). They maybe changed some comma or a word here and there.

What's the best practice in peer-review about this scenario? Is this ethical? Can this be considered a reason for rejection?

It's mostly in introduction and material and methods sections and the study overall is novel enough, so I'd be inclined to suggest a self-citation. But I'm curious if there is a consensus about how to handle such cases.
There seems to be no risk of getting credit twice for the same work here, as the papers are fundamentally different. It is possible that more modifications to these paragraphs would make them better suited for their role in the new paper, in which case suggesting that the authors do so is appropriate for the referee. However, rewriting the paragraphs just for the sake of it seems like a pointless endeavour. As a referee, you should not suggest that the authors waste their time on this.