Operating system: Linux Mint 19
Graphics card: GT 630
2.80 (sub 39), branch: blender2.8, commit date: 2018-12-20 23:46, hash: 7a26e930a8c0
It's hard to put it in words. Setting values by dragging all three value sliders at once, and then dragging all three keyframe enablers at their right hand side, the set value changes to something else:
In words: Keyframe the resting position of the default cube by dragging over all three "Animate property"-knobs which are located at the right hand side of the "Location"-value-triplet in [Properties editor/Object/Transform].
Change the frame,
click-hold on the x-location value, drag over y- and z-value in order to change the triplet by dragging them altogether to the right.
Keyframe the new value of x, y and z by dragging down from the upper keyframe knob to the one at the bottom (z-value).
The set values change in a way that the y and z turn to zero.