Related to https://forum.minetest.net/viewtopic.php?f=18&t=14954
Months ago (10th and 15th November 2015) i moved the 'trees' and 'flat' flags from 'mg flags' to 'mgv6 spflags'.
The flags are a little messy at the moment because to maintain backwards compatibility i could not remove those two flags from 'mg flags', although they are no longer documented as being there. So currently in a world's 'map_meta.txt' file those two flags appear in both 'mg flags' and 'mgv6 spflags'.
Hmmmm is making changes to mapgen settings, and at 0.4.15 these two flags will hopefully be removed from 'mg flags' to cleanup this mess. So it is a good idea, although not essential yet, to set these flags correctly in 'mgv6 spflags' to prepare for future changes.
Mgv6 worlds created after 15 November 2015 will not need this edit, as long as you used 'mgv6 spflags' for 'trees' and/or 'flat', if unsure you should check.
Edit 'worlds/worldname/map_meta.txt' in a text editor.
Find the two lines 'mg_flags = ' and 'mgv6_spflags = '.
Check the 'trees' and 'flat' flags in each line.
Set them to the desired state ('trees', 'notrees', 'flat', 'noflat') in 'mgv6_spflags = '
and remove them from 'mg_flags = '.
The reason this only affects those using 'notrees' or 'flat' flags is that the new defaults in 'mgv6 spflags' are 'noflat, trees', so those will already be correctly set.