Not all blocks in Minecraft are in inventory I recommend to add a new section in creative mode to have the logo of a command block and contains blocks that can be taken only with commands such commad block, mob spawner, mushroom, duble_stone_slab, load block etc. . and in future log 12 to 15 etc.
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Not a bug and is completely by design.

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We're tracking this issue as MC-918, so this ticket is being resolved and linked as a duplicate.
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