Actual mixed voice is walking a thin line between falsetto and chest voice, keeping a precarious balance between elements from both.
You need solid control of both voice types you are blending, and then solid control of the blend. Solid control of both voice types takes years to establish, and now you assume that having found mixed voice for the first time (for the record, I consider it rather unlikely that you actually work with mixed voice here: it's more likely just falsetto) without working hard at it by traversing the vocal break from above, namely falsetto, and it should sound great? The most complex singing feat of all should just be there, ready to be "discovered" rather than developed, and be useful?
There is a reason professional singers take decades of lessons, you know. Mixed voice is not something you "discover", like you don't "discover" how to juggle. It's the result of a conscious blending of two voice types you are in control of, and that blend does not happen by accident since its in the middle of two stable configurations of the larynx. It sits in the middle of what happens when your voice flips. You stay stuck in the middle of that flip for a range covering several notes.
For this to make sense, your chest voice must be well-controlled and sounding strong, and your falsetto must be well-controlled and sounding strong. If either has still problems, hanging in the unstable middle between them will have more problems.