The y coordinate will show -0.00000 in the "XYZ" line and -1 in the "Block" line of the F3 screen if you jump without moving sideways and land on y=0 (i.e. on a full block at y=-1). It resets to 0.00000 and 0 after moving sideways.
Before jumping:
[media]After jumping:
[media]
Comments 2
Seems to be fixed in 1.18 Pre-release 6, probably by the fix for MC-238959
Just an inconsequential rounding artifact