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

The bug

The spawner is completely absent from the Creative inventory for some reason. It would be expected it would be there, as it isn't a very technical block like barriers or command blocks which have been intentionally kept out of the Creative inventory.

This is also a parity issue, as the spawner does appear in Bedrock Edition's creative inventory. The fact that it exists in the Creative inventory in Bedrock Edition may be due to some behavioural differences; placed spawners are empty by default in Bedrock Edition, but spawn pigs in Java Edition, so it may be advisable to make Java Edition's behaviour match Bedrock Edition's.

How to reproduce

1. Give yourself a spawner: /give @p minecraft:spawner
2. Note the item name - this will be used for searching
3. Open the Creative inventory
4. Enter the Search tab
5. Search for Spawner
6. See no relevant results