The bug

If there a multiple possible command arguments and one is also the prefix of other ones, no further suggestions are shown when that argument is typed.
E.g. the block argument "sand" is also the prefix of "sandstone", "sandstone_slab", ...

How to reproduce

1. Type the following

   /setblock ~ ~ ~ sand
   

   As soon as the "d" of "sand" is typed, no further suggestions ("sandstone", "sandstone_slab", ...) are shown and they cannot be tab completed either