This affects both multiplayer and singleplayer.

I discovered this using a custom command, but it works for vanilla commands too.

Enter "/enchant @e minecraft:aqua_affinity" into chat, press space.

you see: error: 

I expect: the parameter hint for the following argument.

Second test:

Enter "/enchant @e minecraft:aqua_affinity" into chat, press space twice, press left.

you see: param hint: 

I would expect this param hin to be displayed without the second space, when there are no suggestions available.

This is reproducible both with required and optional arguments.