When a player is hit by an other player the nbt tag Motion[0] and Motion[2] aren't tracked this is not the case when an entity hits a player.
scoreboard objectives add motionx dummy
execute store result score <playerName> motionx run data get entity @p Motion[0] 10000
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We're tracking this issue as MC-122814, so this ticket is being resolved and linked as a duplicate.
That ticket has already been resolved as Won't Fix, which means this is considered a bug but won't be fixed. The description of that ticket or the comments might explain the rationale. Please do not leave a comment on the linked ticket.
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