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.
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.