Sometimes you wonder what you've sent earlier, or you need again something that you already wrote earlier. But if you wrote a command, depending on how it ends, when you get back to it, it can suggest something, making the up and down arrow keys switching between these suggestions (so what they do suddenly change).
You can hit escape to close the suggestion box and continue what you were doing first, but it can quickly become annoying.
What I've noticed to do that (but I'm pretty sure it happened to me for other things as well) are target selectors and potion effects. So this willl happen, for example, for these completely valid commands :
/kill @e[type=!player]
/effect clear @p minecraft:fire_resistance
Linked issues
Comments 2
But I don't want to disable command suggestions, they just don't need to interfere when you review messages you've already sent (forgot to check if there was an option for that though, now I know there is so still thanks for that). This is something else than having suggestions popping up when you're actually currently writing your command. If you're not, that it's something you already wrote earlier, it's useless, so just annoying (and if you do want to modify that command, you can do like for other arguments that do not make their suggestions pop up, like the "true" in "/effect give @p minecraft:potion_effect 100 4 true", or the block in "/setblock ~ ~ ~ minecraft:block_name")
And I don't know what "WAI" means. After a little thinking, I'm guessing that stands for "Works As Intended". But I truly doubt that. Typically, why should that work differently with potion effects and blocks ? Because it does, but it doesn't make sense. It should either always be the case (which would be a bad decision though), or never (in the situation I depicted only. Not talking about when you're writing a command).
WAI.
Disable command suggestions in:
Esc > Options > Chat Settings > Command Suggestions > OFF.