Why does Bitcoin block 526,591 have a 6.25 (instead of 12.5) block reward? https://chain.so/block/BTC/0000000000000000002ba5a1fb96f93e6c215d62db4280f1cbd30e82c7c71fba
https://blockchair.com/bitcoin/block/526591


It's an empty block (zero transactions), but the coinbase transaction is 6.25

The blockchain.com explorer shows this as having a -6.25 fee (despite having no transactions) https://www.blockchain.com/btc/block/0000000000000000002ba5a1fb96f93e6c215d62db4280f1cbd30e82c7c71fba
There is a maximum limit on the block reward which is 12.5 BTC but nothing prevents a miner from claiming less than 12.5 BTC. In fact, there have been times when miners forgot to claim any bitcoin at all (claimed 0 BTC), a very expensive mistake. This is probably a mistake from the miner, he could have certainly claimed more bitcoin. 

As long as the block is valid, it will be accepted by the network, even if it's unfair for the miner. In practice, this means that the maximum supply of bitcoin is not 21 million BTC but 21 million BTC - unclaimed BTC by miners. People often see this difference by making the difference between sum(number_of_block_i * block_reward_of_block_i) and the total bitcoin counted from the UTXO set.