For a comprehensive list of other examples of issues with item positions in the Creative inventory, click here.

All tickets associated with item placement in the Creative inventory are valid for cases introduced in 1.16 and later as per this comment. Exceptions exist, which are valid for their own reasons.

The bug

Shroomlights and wart blocks both grow as constituent blocks of huge fungus canopies. Despite this, they aren't grouped very closely at all in the Creative inventory, despite what one may expect.

How to reproduce

1. Open the Creative inventory
2. Locate Shroomlight in Decoration Blocks
3. Locate Nether Wart Block in Building Blocks
4. Locate Warped Wart Block in Building Blocks
5. Note that these don't even all share the same tab