When pressing the command key (the one that opens the chat with a pre-written /), autocomplete doesn't open (to suggest words like advancement, bossbar, clear, …).
Deleting and re-typing the slash doesn't open autocomplete either. Writing something after the slash and deleting it again also doesn't open autocomplete. Only pressing the chat key and manually typing the slash opens autocomplete.
This was caused by the fix for MC-122596. A better solution than working around all these inconsistencies and bugs would be to just make arrow keys not go through suggestions. Tab and Shift+Tab are enough. (Or mouse, haha.)
Press TAB or type anything to bring autocomplete.
And personally, I think removing option to scroll with arrows would be annoying, mostly due to muscle memory (every IDE I used worked that way - if widget is in focus, it responds to arrows).