How to reproduce: Type in
/effect @p invor
/enchant @p sharpand hit tab, nothing will happen. What should happen is that "inv"/"sharp" should be completed to "minecraft:invisibility"/"minecraft:sharpness", just like
/give @p stontab-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.