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
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
Can confirm that this is still an issue in 20w51a.