Newton's laws hold only in inertial (i.e. unaccelerated) frames. The
acceleration of the block is the same in all inertial frames (Galilean
Relativity). Thus all inertial observers agree on the direction of the
acceleration (let's say towards -x). In the first frame where the
initial velocity is towards +x the block is slowing down, the
displacement and force are opposite, the work done is negative and the
KE decreases.
In the 2nd frame (from the first observers point of view moving toward
+x FASTER than the initial velocity of the block), the initial velocty
is towards -x, as is the force, the displacement and force are parallel,
positive work is done, speed (toward -x) increases and KE increases. The
block appears to be in contact with a moving medium (the desk which is
also moving toward -x) and will come to a final speed the same as the
moving desk.