B'reshit 49:10 was the first text discussed in the Disputation of Barcelona (July 20-24, 1263), a formal debate ordered by the king between representatives of Christianity and Judaism regarding whether the Moshiach had come. This indicates how important this passage was.
לֹא-יָסוּר שֵׁבֶט מִיהוּדָה, וּמְחֹקֵק מִבֵּין רַגְלָיו, עַד כִּי-יָבֹא שִׁילֹה, וְלוֹ יִקְּהַת עַמִּים.
Here occurs the puzzling term shiloh (שִׁילֹה), which the Leeser Bible (1853), agreeing with the 1611 King James Bible of Christians, read as a name of Moshiach (but the New Jewish Publication Bible says: 'Meaning of Heb[rew] uncertain'). Onkelos in his Aramaic translation read this word as shelloh (so Rashi), the archaic form for his, 1 understanding a segol in the first syllable and no yod: '. . . until Meshicha comes, whose (דְּדִילֵיהּ הִיא) is the kingdom':
לָא יִעְדֵּי עָבֵיד שׁוּלְטָן מִדְּבֵית יְהוּדָה וְסָפְרָא מִבְּנֵי בְנוֹהִי עַד עָלְמָא עַד דְּיֵיתֵי מְשִׁיחָא דְּדִילֵיהּ הִיא מַלְכוּתָא וְלֵיהּ יִשְׁתַּמְעוּן עַמְמַיָּא׃
The Masoretes, on the other hand, transcribed this word with a chireq and yod in the first syllable.
Why is 'shiloh' read as if it were transcribed 'shelloh' by Onkelos in Genesis 49:10 when the Masoretes, who give the traditional reading, transcribed it differently?
In order to avoid merely personal opinion, I am asking for medieval and modern commentators. The question does not concern the Hebrew, but rather the way the text was allowed to be treated.
See my recently written answer to a related question for more on the term: What's the Meaning of Shiloh in the Last Blessing of Yaakov on his Children?
1 R' J. H. Hertz, The Pentateuch and Haftorahs, p. 202: 'It is a strange circumstance that the older Jewish versions and commentators (Septuagint, Targums, Saadyah and Rashi) read this word without a 'yod,' as if written 'shelloh' the archaic form for 'his'; or as if it were a poetic form for 'peace.'