When you modify an item entity's motion value, after the initial summon, the new motion is not applied visually, even though the item still ends up in the same place.
Until the item stops completely on the ground, the item travels according to the original Motion[] values, but when it lands, it teleports to the position of the new Motion[] values.

Shown here: GIF link
These are the 4 commands in the command blocks:

summon minecraft:item ~ ~2 ~ {Item:{id:"minecraft:redstone_block",Count:1b},Tags:["newitem"],Motion:[0.1d,0.2d,0.1d]}
data modify entity @e[tag=newitem,limit=1] Motion[0] set value -0.1d
data modify entity @e[tag=newitem,limit=1] Motion[2] set value -0.1d
tag @e[tag=newitem,limit=1] remove newitem

Note: This is on singleplayer, I haven't been able to test on multiplayer yet.