Gamerule no longer working

I did not know how to search using 'Date created', so:

I've used this gamerule command to, for example, prevent grass from dropping seeds:

/gamerule DoTileDrops false

It does not work. Breaking tall grass still gives seeds. In another saved game, this command seems to work but not in a recent new game I created this morning.

So I tried the following command to be sure I wasn't making the wrong/right one:

/gamerule DoEntityDrops false

No effects.