Long story short, and this is a topic I have read and researched extensively, Assyrian Church of the East is neither Nestorian in nature, nor has it ever fully adapted his teachings.
The long answer? it all has to do with history and the context back then. I am talking 4th and 5th century, when the initial Christian councils were being held, specifically that in Ephesus (431 AD)
The Assyrian Church of the East is often linked to Nestorianism, but many scholars and church members consider this label misleading and frankly confusing, as seen by all those who ask whether the Assyrian church is considered Nestorian. In fact, some members of the church itself ask this question. Nestorius, a 5th-century bishop, emphasized the separate divine and human natures of Jesus Christ. His teachings were deemed heretical at the Council of Ephesus (431 AD). Despite this, some Eastern Christians, especially in Persia, adopted similar views due to political and theological differences with the Byzantine Empire. This is why I mentioned that historical context is important here. In other words, due to the patriarchal seat of the Assyrian Church of the East being situated in Persia, which at the time was in conflict with the Roman/Byzantine empire, Byzantine Christianity, representing the Roman empire, wanted to tarnish the image of Eastern Christianity, based out of Persia and Assyria.
The Assyrian Church has its own distinct theology, even if some of its belies and doctrines may be similar to that of Nestorianism.