How to reproduce:
1. Create a new project
2. Create a cube in the scene and move it so x, y, and z are not equal to 0
3. Create a new animation
4. Add transform position property
5. Drag red time indicator to first animation keys
6. Delete Position.y, Position.z keys
7. Repeat step 5 and 6 only in last animation keys
8. Play the animation

Actual result: Animation keys are reset to default value and that value is 0
Expected result: The default value falls back to the transform's actual default value

Reproducible: 5.3.6p5, 5.4.1p1, 5.5.0b4

By Design: The default value for non-animated position/rotation/scale curves respectively is 0/0/1. As the tooltip states, you can't animate partial rotation, position or scales.