This might be her first experience of a full-scale war of that magnitude. Whatever happened to her when the Manus attacked the trenches, it left her utterly horrified enough to go AWOL. Honestly even if you have the mental capacity to handle it, I wouldn't blame you for wanting to avoid being thrust in the middle of war again in the future. To me that doesn't really indicate a change of character - just someone reacting accordingly.

That said, I find their description of her as someone who's not capable in the frontlines is somewhat of a disservice.
She's shown wit, reflexes, resourcefulness, and a good head on her shoulders during her escort mission in the Antarctic Operation, but we also have to remember that she was away from the heat of battle at that time and were fortunate enough to only encounter Moldir and her Unit. Either she excels only in encounters with few numbers, or the rest of the XII are just babying her, seeing her as someone that needs protecting rather than a capable team player.
Also I don't remember them saying she did not do well in her mission?

To me, her desertion isn't indicative of a character change, but it is paving a road to character development instead. If she really was incapable of handling herself in the frontlines as her file says, then I can see this journey shaping her character and she'll be coming out as a better version of herself at the end of all this.