Strange nikkud in the Shadal siddur

In 1821 the Italian Jews were required by the Austrian government to produce a translation of their siddur and, through circumstances I described here, Shadal - only 20 years old - was selected to do the translation. It appeared in Vienna the same year. Here is the title page (1829 printing):

I noticed something interesting - with the caveat that I am not at all sure that he had anything to do with the Hebrew text printed therein - but have a look at the nikkud of the word רבי in these excerpts from the Mishnah:

As you can see, for some reason some of the rabbis are ribbi, with a chirik, and some rabbi, with a patach, and in the same section. Why? Careless editing?

I wonder if Shadal was responsible for this pointing, and if so, he must have had his own chiddush in mind about the proper way of pointing רבי, as I quoted in this post. The traditional pointing was generally with a chirik and never with a patach. Some grammarians were able to adduce evidence for chirik, but the reasons for a patach were always etymological, and could never be deduced from early sources. Apart, I might add, from the Greek of the New Testament, where the word rabbi is spelled with an alpha. However,   in the 18th century Isaac Satanow argued successfully for rabbi with a patach, probably because he was followed by no less an authority than Wolf Heidenheim. It is this pointing which eventually won the battle of the nekkudot. However, Shadal adopted a middle position. While he agreed that the textual evidence supported ribbi, he concluded that the arguments for rabbi were compelling. Therefore, he conjectured, the original pointing could have been rabbi, and it was reserved for rabbis with true ordination. However, when the term began to be used by unauthorized people, the title was modified to ribbi, and that became the mundane title for non-ordained scholars.

So we see here that the title of the early Tannaim are pointed rabbi, while a later one, like Rabbi Yose, is pointed ribbi. The only problem I have with this is that obviously Rabbi Elazar ben Azaryah, Rabbi Elazar and Rabbi Chanina are not later Tannaim (Rav Zeira was an amora, but he lived in Eretz Yisrael). Maybe experts in rabbinic chronology and/or who the chokrim believed were ordained and not can enlighten us.

Of course it can all be one giant mistake.