This is a message shown in the bundle download link:
"Projects in this bundle are hidden in your library by default until you first access them in order to avoid flooding your library. You can return to this page at any time to access any projects you wish to show in your library."
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Also, this may be of help:
https://medium.com/@stadja/auto-claim-your-itch-io-mega-bundle-games-c425f6a9c1a...