There are few spoilers on how that is possible. One spoiler is she’s lying about the future. She’s trying to fix the mistakes, making multiple timeline. According a spoiler that backs up this theory is Tia goes into a 3 month coma, and in that coma she experiences the bad timeline.
Another theory is, as she had mentioned in the story, she can make small changes that won’t make a huge butterfly effect or ripple. If it’s small details, that no matter what happens it will still have the same outcome. An example of this her becoming the youngest commander like her bio father was when he was younger. It’s also on the people in the future who are with the younger version of Tia to not reveal too much information. Like who her future husband and emperor going to be. The only thing we and Tia know is she marries one the Princes, but he made sure the only thing she’s is his hair. Not even his eyes since, the first and eighth prince have amber eyes, second prince has blue eyes and sixth prince has green eyes, we don’t know about the other two princes.
I like that the premise is unique but how can the mc go back and forth in time and not affect the timeline?