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I think this would be best to report to the Tinkers Construct github: https://github.com/SlimeKnights/TinkersConstruct/issues

Although, you might want to try to create your own tiny modpack with just the latest 1.16.5 versions of tinkers construct and mantle and see if you can replicate the issue before reporting it to them. Also, make sure there's not any other upgrades you have missed that can add extra abilities/upgrade slots (but I can't think of any).

If you can make it unbreakable with just tinkers construct and mantle, but not in Create: Above and Beyond, then this issue is the one to keep.