For example the following dataset:
https://huggingface.co/datasets/allenai/c4
Lacks a Croissant tag, not just in the UI but also if filtering by "library:mlcroissant" with the API. However, the Croissant file is available in the API:
https://huggingface.co/api/datasets/allenai/c4/croissant
When looking at the 15k most download HF datasets, around 4k were lacking this tag. Sometimes this might be justified due to a faulty DatasetInfo, but that's not always the case as we have seen with allenai/c4.
fyi @lhoestq