That would mean, though, that other TTL flashes are being overridden in some way--or their TTL information is ignored--by the setting for the master flash.
I don't think its ignored, just that a small amount of compensation is added.
Unfortunately I don't know how the conversion from TTL to a Manual power level is done to start with, so I'm not sure why using different flashes causes an issue then either.
Its not completely out though. I mean there's a number of stops in power difference between a V860II and AD600 for example, so if it made the power level equal for both for example then the result would be a long way out, not just a little bit.
Anyway you guys can experiment with it and might be able to make more sense out of it. Maybe it could be further refined by the time they ship as well. Just letting you know what I can see so far.
If it hadn't have been working well from the start using the same flashes then I would have been concerned. Knowing the custom function was there for when mixing strobes I pretty much figured there was going to be more involved in that area.