Warning: This article contains spoilers for Stranger Things season 4, Vol.1
Largely considered a plot hole since Stranger Things season 2, Kali's character could actually be explained by Vecna's powers. Stranger Things season 2 revealed that Eleven was just one part of a cog in the Hawkins Lab machine, as there was a whole host of children with superpowered abilities being hidden away. One of them was Eight/Kali, who was friends with Eleven in the Lab when they were young children. The two reconnected for a short while before Eleven left again for Hawkins to save her friends, and during their brief time together Kali created some possible plot holes.
In the opening episode of Stranger Things season 2, Kali's powers are shown to manifest in conjuring incredibly lifelike visions and hallucinations, which she uses to aid her criminal activities. This is, of course, vastly different from what audiences had previously seen in of Eleven's telekinetic powers. Then, in their conversation, Kali mentions that one day she went to the Rainbow Room and Eleven had ''gone,'' which suggests that El escaped before Kali did. This created extra confusion in Stranger Things season 4 with the Hawkins Lab massacre flashback, which Eleven was present for, but Kali wasn't. Thus, Kali's powers, and the timeline, were thought of as void plot points in the series, but perhaps they can be explained by Vecna's powers.
Stranger Things season 4 revealed that all of the test subjects are the result of Dr. Brenner's attempt to replicate Henry Creel's powers, making him Number One. Number One, later known as Vecna, has both telekinetic powers and can produce lifelike visions, shown in the flashbacks of the Creel family in the 1950s. Thus, when trying to replicate his powers, test subjects could have gotten one power or the other (or both, which is yet to be seen), which is why Eleven has telekinesis and Kali can conjure visions, explaining the plot hole of Kali's different powers. Then, upon the test subjects' powers manifesting in different ways, they could have been split into one of two different groups to better focus their powers. This would then explain how El and Kali became separated and explain the timeline.
In the flashback of Kali and Eleven as children in the Lab, they are both relatively young. They aren't wearing hospital gowns, and neither has a shaved head, thus it's possible that their Kali isn't in the flashbacks of the massacre; she had already gone.
Stranger Things has placed a lot of focus on Eleven's powers and her time in the lab with her telekinetic brothers and sisters, which has left a lot of Kali's past in the dark. It could be that the test subjects with visions were simply not important to Brenner, as their powers were not helpful in spying on Russians, thus they haven't been focused on in the show. More sinisterly, test subjects with visions could have been eliminated entirely after Kali's escape, hence why audiences have only seen Kali with these powers. Not only were her visions purposeless in of spying, but they also made her a flight risk, putting the whole operation in danger of exposure. However, with the official introduction of Vecna, and the beginning of Brenner's experiments being explored, Kali's history could become clearer and tie up any loose ends in Stranger Things.