How to reproduce: Type in
/effect @p inv
or
/enchant @p sharp
and hit tab, nothing will happen. What should happen is that "inv"/"sharp" should be completed to "minecraft:invisibility"/"minecraft:sharpness", just like
/give @p ston
tab-completes to "minecraft:stone".
I believe this is related to when plugins provide their own effects and to not confuse which effect is being applied.