When mixing animations on different tracks in Spine, disappearing elements can often be attributed to how animations are layered and how their timelines are applied. Each track in Spine is applied sequentially, with higher tracks overriding the poses of lower tracks. If an element disappears when you add a new animation on a higher track, it might be because the new animation does not have keys for that element, causing it to revert to its setup pose or become invisible if not explicitly keyed.

To resolve this, ensure that the animations on higher tracks have keys for all elements you want to remain visible. Alternatively, you can use an empty animation to mix out from the last frame of the previous animation, or adjust the mix duration to ensure smoother transitions. Additionally, check if any attachments are being set to invisible in the animations, which could cause them to disappear when the animation is applied.