It could be she's at a "stage" in her bhs where she's got too much going on as she does the skill to put form corrections to use. Another likely......She thinks the skill is done more easily with bent knees, and won;t go about working on form until she's feeling solid on the skills. The problem with that logic is that form isn't just a means to a higher score. Straight legs during a bhs make the skill easier, here's the short list.....
A bhs requires a strong jump backwards into the skill, and the follow through to a strong jump is extended/straight legs. Weak jump leads to a weak follow through (bent knees)......a weak jump with bent knees means less "power" to work with, and that usually results in a genuine need to bend the knees, making the skill look even worse.
Bent knees as her hands hit the floor will take away her ability to push up and off her hands to a standing position. I've yet to see, in my lifetime, a "pogo stick" that bends somewhere in the upper half to help it go higher. Maybe if I stick around another 20 years, some opportunistic inventor will come up with a "new and improved" pogo stick that bends in mutiple locations, but until that day you can assume that straight and tight bodies will bounce better than bent and loose ones.