The first full trailer for Natalie Portman become Lady Thor, marking not just the character's ascension to hero status but also her return to the MCU. After Thor: The Dark World, Portman was famously reported to have cut ties with Marvel and the MCU, and other than Jane Foster being briefly glimpsed in Avengers: Endgame, this appeared to be true.
However, Portman is set to reprise the role in Thor: Love and Thunder, assuming the mantle of the Mighty Thor in the process. An early trailer for the film showed Portman as Foster wielding a reforged Mjolnir, and the full trailer sees her briefly interact with Hemsworth's Thor. The pair discuss their relationship and touch on Portman's absence, referencing their in-universe break-up.
Thor and Jane Foster's break-up was referenced in ing in Thor: Ragnarok, but the Thor: Love and Thunder trailer mentions it more directly. Exactly what happened to Jane Foster between Thor: The Dark World and Love and Thunder will undoubtedly become clear, but the trailer confirming that the characters will acknowledge their break-up cements its place within MCU canon. Thor detailing the exact time since they last saw one another also seemingly confirms two other details: an approximate timeline for their break-up, and that it was Jane's decision (and not mutual, as he comically insisted to a fan in Thor: Ragnarok).
In the trailer, Jane asks Thor how long it has been since they last saw one another, and Thor answers that it has been "8 years, 7 months and 6 days... give or take." This implies that Thor is far more upset about their break-up than she is, but also references Thanos' Infinity War snap, as Foster believes it has been "3 or 4 years". The exchange evidences their different perceptions of the break-up, too, with Foster having only an approximate idea of the time frame where Thor appears to have been counting the days.
Though the trailer doesn't offer any specific details on the cause of their break-up, it does seem to confirm that it was Jane's decision. This may be a subtle meta-reference to Portman's own departure from the MCU, which was well-publicized after Thor: The Dark World. The timeline offered by the conversation in the trailer establishes that the break-up happened sometime shortly after the events of Avengers: Age of Ultron, but the exact details have yet to be explored. However, it would seem that what went wrong between Jane Foster and Thor is most likely tied to Thor's status as an Avenger and a god who only comes to Earth when duty compels him, which had been seen to cause tension between the pair leading up to Thor: The Dark World.
The exact reason for their break-up is hardly even hinted at in previous MCU movies, as the nature of Portman's exit from the MCU made it a difficult subject to broach directly. That Thor: Love and Thunder will explore the in-universe reasons for their break-up is an exciting development alone, and it's one that previously seemed unlikely to ever happen. Though the specific reason that Jane broke up with Thor hasn't been fully disclosed, the Thor: Love and Thunder trailer suggests that she may not have found their break-up as difficult as Hemsworth's God of Thunder.
