I guess the small free space normally available in a block is being "spammed". Wonderful
free space is a thing of the past, unfortunately. Another one of satoshis ideas getting buried.
0.12 nodes don't even relay 0-fee tx.
Free transactions are nice and we can keep it that way if people don't abuse them.
Well, guess what happened.
As for 0.12, from what I understand that's under max mempool limit (which you can set as high as you want, depending your node's ram capacity).
Bitcoin Core has a heuristic 'priority' based on coin value and age. This calculation is used for relaying of transactions which do not pay the minimum relay fee, and can be used as an alternative way of sorting transactions for mined blocks. Bitcoin Core will relay transactions with insufficient fees depending on the setting of -limitfreerelay= (default: r=15 kB per minute) and -blockprioritysize=.
In Bitcoin Core 0.12, when mempool limit has been reached a higher minimum relay fee takes effect to limit memory usage. Transactions which do not meet this higher effective minimum relay fee will not be relayed or mined even if they rank highly according to the priority heuristic.