This has been reported at least twice:

How do I turn off the tooltip for certain item? I already got the mounts so I no longer need the tooltip.

Source: Discord

how do I remove this ? I already dropped 10 so I don't need to see it anymore

Source: Discord

It's unclear whether this is due to the recent tooltip changes, or by design. Looking at the code, it might be the latter?

It seems that the original design accounted for learnable items (mounts and pets), which are indeed hidden via this setting:

However, the shards aren't collectibles and can't be learned, so the setting won't work. A new one can be added, though:

There's some overlap and potential for conflict here, but known items should be disabled automatically, unless set to repeated.