Months ago i moved the 'trees' and 'flat' flags from 'mg flags' to 'mgv6 spflags'. I tested at the time and it was coded to not break compatibility, however i have discovered that old mgv6 worlds with 'mg flags = notrees' now have trees enabled. Funny that no-one seems to have noticed or complained, i guess that, luckily, few worlds had trees disabled.
Compatibility for the 'flat' flag was not broken.
Sorry for any problems caused by this.
The flags are in a mess at the moment because in my effort to maintain backwards compatibility i did 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'.
So here's how to fix your old mgv6 world if you now have unwanted trees:
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, they are probably correct in 'mg flags' but may be incorrect in 'mgv6 spflags'
Set them to the desired state ('trees', 'notrees', 'flat', 'noflat') in both lines.
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. Anyone with old mgv6 worlds which are flat or have trees disabled are encouraged to do this.