I too would like to know this and be able to prevent the on command following the brightness one.
I’m just setting up a simple light via MQTT that only has a brightness, so on = 100, off = 0.
The payload for on is the same as setting the brightness to 100.
This is a problem, since setting brightness to, say, 50 is followed by on, which sets it to 100.
FYI I think part of this is fixed. See the PR #9829 for details. You can set the light to output just the brightness now (or change when the on command is sent).
However, I’m still having issues with this behavior in that I can’t get any of the 3 MQTT lights (regular, JSON, and template) to accept the just brightness back as the state (see Issue 7810 if you’re interested).