I have this problem too. They seem to mismatch the .attr +l (halfop) flag (even with an all 1.6.17 net) to the .botattr +l (leaf). Then the slaves start fighting with each other and the hub, usually disconnecting and stating that the hub has unleaflike behavior.
This is especially annoying because I don't have any bots specifically set to leaf in the first place. So I'm still stuck with a 1.6.15 network.
Also, the +l flag will still be there even if I set .attr -l or .botattr -l on any or all 1.6.17 bots. If .die is issued and the +l is then manually removed from a userfile, they will automatically +l again upon restart.
I should also probably note that this only starts to happen after more than 2 bots are connected to each other in a mixed network (1.6.15 and 1.6.17). I don't believe I've tried all 1.6.17 yet or, if I did, I don't recall what the result was.