In Arrival the aliens changed Louise's perception of time. So she only should've had visions of Hannah (her future daughter) after the aliens taught her this new way of seeing time. But in the movie, we see this even before they arrived. I might be completly wrong. But I'm really confused.
On one hand, you could consider what Louise was told- that with this alien language, time becomes unapparent and non-linear, thus she is able to perceive (almost accidentally at times, it seems) events in her life before she even realises they are indeed her life.
Alternatively, an opening soliloquy is not an unusual movie trope and this is one done very well in Arrival, which could either be a product of the end of the movie (coming, ahem, full-circle) or simply a filmmaking technique used to hook and draw us (and Louise) into her story.
And it does. Compare this with the first 10-minutes in the Pixar movie Up. Although this is an in-movie remembered-event, the time frame within those first ten minutes are offered as a spectacle, as something to harness your interest, but also builds character faster than a description or an explanation. In fact, I wouldn't be surprised if this was something of an inspiration for this intro sequence as Up gained a lot of word of mouth advertising with regards to those first ten minutes.
The movie begins with Louise telling her story (to her child, or to herself, or to the audience), so, in order for this to be feasible, this should happen post-Aliens events, so her perception of time (and consequently our own perceptions) will be convoluted as time goes on.
Basically, because the events of the movie are only known from Louise's perspective, we are experimenting with time-continuity as she does.
Or/and, as Gray Roberts points out, this could be a cinematic strategy. Denis Villeneuve already used this kind of trick in his movie Enemy (2013).