Actually it is possible to create a coinbase transaction which pays less than the maximum possible amount.
Check this url about protocol rules
https://en.bitcoin.it/wiki/Protocol_rules which explains all the checks which are done to ensure consensus.
...
"block" messages
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15. Add block into the tree. There are three cases: 1. block further extends the main branch; 2. block extends a side branch but does not add enough difficulty to make it become the new main branch; 3. block extends a side branch and makes it the new main branch.
16. For case 1, adding to main branch:
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2. Reject if coinbase value > sum of block creation fee and transaction fees
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To verify this statement I was also checking the bitcoin code what is actually implemented and I found this
https://github.com/bitcoin/bitcoin/blob/master/src/validation.cpp#L2011-L2017 bool CChainState::ConnectBlock(const CBlock& block, CValidationState& state, CBlockIndex* pindex,
CCoinsViewCache& view, const CChainParams& chainparams, bool fJustCheck)
{
...
CAmount blockReward = nFees + GetBlockSubsidy(pindex->nHeight, chainparams.GetConsensus());
if (block.vtx[0]->GetValueOut() > blockReward)
return state.DoS(100,
error("ConnectBlock(): coinbase pays too much (actual=%d vs limit=%d)",
block.vtx[0]->GetValueOut(), blockReward),
REJECT_INVALID, "bad-cb-amount");
...
You can clearly see, that it's not allowed to reward more than the block reward + transaction fees, but anything equal or smaller than the block reward + transaction fees is fulfilling the consensus rules and is valid.
The question is just, if and why someone would not claim the maximum possible reward when mining a block.
Nevertheless that this situation is possible, I'm not sure if your example is actually doing this or if there is another reason like paxmao or teramit already explained.
If you could provide the block hash of your example we could give you a better reason what exactly happens with the block reward inside this block.
