Some Shi'i scholars have accepted the idea of deletion or rearrangement of material from the Qur'an based on narrations.
Some Sunni narrations also discuss deletions of material in the Qur'an, so it is not a Shi'i-only subject.
Most Shi'i scholars today reject those narrations and reject the idea of tahrif in the Qur'an. You can find a good argument and overview of the narrations and the subject in https://www.al-islam.org/al-bayan-fi-tafsir-al-quran-prolegomena-quran-s...
This can be a very sensitive and taboo subject because it is generally taken as a primary pillar and fundamental belief of Islam that nothing in the Qur'an has ever been changed (apart from the minor variations in qira'at).
Shi'is are also particularly sensitive because they feel defensive because they feel they have to "prove" to Sunnis that they have the same Qur'an. (And, in fact, Shi'is do use the same Qur'an and same recitations as Sunnis.)
However perhaps it's good to step down from the ideologically and sectarianly charged nature of this discussion and just take it as it is. If some people held this view, it was their view, and that is that.