See https://forum.minetest.net/viewtopic.php?f=18&t=16595 for more details.
This PR is now being merged.
"To introduce the new flags I had to increase the map block serialization version, this is why you can not play on your maps on other versions of Minetest after you opened them in this version.
Of course old clients still can play, just the server has to be new to be able to read the map."
This includes singleplayer.