Does the block reward comes from a bitcoin treasury? If yes, then how is it issued?
There is no treasury. The Bitcoin is generated out of nothing. The coinbase transaction has special rules. It is allowed to have only one input which has no previous output and really no value. It is allowed to create outputs which have a total value of the block subsidy (currently 12.5 BTC) plus the transaction fees from that block. Those coins aren't issued by any entity; the miner just creates the output(s) and is allowed to do so. This rule is enforced by all of the full nodes on the network who will reject that miner's block if he pays himself too much money.

Simply put, the miner is allowed to just pay himself money that is produced out of nothing.