Why do block explorer APIs say the nonce in the block header is 0? Several block explorers are reporting that the nonce value in many block headers is 0. For example, blockcypher,

Why does this happen?

Where can I get the actual block headers?
This appears to be a bug in their software (i.e. it occurs for no reason other than it's a mistake) so you should report this to Blockcypher.