When making custom blocks in addons, when you have "register_to_creative_menu": true" the blocks that you make doesn't appear in Creative Mode. You have to remove that part of the Json file in order to get them but you can only get them with commands.
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We're actually already tracking this issue at MCPE-63121, so I resolved and linked this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
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