I haven't investigated, but you may be seeing segwit-invalid blocks...0.13.0+ nodes will enforce segwit as it activated some time ago on testnet, 0.12.X nodes will not.
On March 23, 2017 3:37:34 PM PDT, Juan Garavaglia via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote: >We notice some reorgs in Bitcoin testnet, while reorgs in testnet are >common and may be part of different tests and experiments, it seems the >forks are not created by a single user and multiple blocks were mined >by different users in each chain. My first impression was that the >problem was related to network issues but some Bitcoin explorers were >following one chain while others follow the other one. Nonetheless, >well established explorers like blocktrail.com or blockr.io were >following different chains at different heights which led to me to >believe that it was not a network issue. After some time, a reorg >occurs and it all comes to normal state as a single chain. >We started investigating more and we identified that the fork occurs >with nodes 0.12; in some situations, nodes 0.12 has longer/different >chains. The blocks in both chains are valid so something must be >occurring in the communication between nodes but not related with the >network itself. >Long story short, when nodes 0.13+ receive blocks from 0.13+ nodes all >is ok, and those blocks propagate to older nodes with no issues. But >when a block tries to be propagated from bitcoind 0.12.+ to newer ones >those blocks are NOT being propagated to the peers with newer versions >while these newer blocks are being propagated to peers with older >versions with no issues. >My conclusion is that we have a backward compatibility issue between >0.13.X+ and older versions. >The issue is simple to replicate, first, get latest version of >bitcoind, complete the IBD after is at current height, then force it to >use exclusively one or more peers of versions 0.12.X and older, and you >will notice that the latest version node will never receive a new >block. >Probably some alternative bitcoin implementations act as bridges >between these two versions and facilitate the chain reorgs. >I have not yet found any way where/how it can be used in a malicious >way or be exploited by a miner but in theory Bitcoin 0.13.X+ should >remain compatible with older ones, but a 0.13+ node may become isolated >by 0.12 peers, and there is not notice for the node owner.
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