As of 1.21.5, wind charges that are summoned using the command “/summon wind_charge” do not keep their specified velocity.
To replicate, summon a wind charge anywhere with a low velocity using “/summon minecraft:wind_charge ~ ~ ~ {Motion:[0.1d,0d,0d]}”. Notice that the wind charge significantly speeds up beyond the specified motion values shortly after summoning.
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Wind charges do not experience drag, so summoning them with acceleration_power:0 causes them to fly according to their Motion. Why that field defaults to 0.1 for them I do not know. Why it even exists I do not know, since its only purpose is to counteract drag on small fireballs and projectiles derived form them and it would have made much more sense to just not have those projectiles experience drag in the first place. Regardless, I think it makes sense to track all the acceleration_power-related issues in MC-80142, so I will request that this report be resolved as a duplicate.
As a side note, this bug and “MC-279003-Explosions do not affect wind charges” are preventing me from releasing a major parkour build that I spent 4+ years overall making. I hope these two issues can be prioritized, as they directly affect gameplay.