Is brightness a required value when setting transition for light services?

I’m working at setting up the flux component along with motion sensors for light control. My hope is that flux (or other automations) could control brightness, with a blueprint to just handle the on / off bit.

I noticed that setting transition in my blueprint wasn’t working. It looks like if I call light.turn_on with transition, but no brightness, the lights (either Hue or Ikea bulbs paired with a ConBee) instantly turn on to full brightness:

entity_id: light.kitchen
transition: 5

If I add brightness, the transition works fine:

entity_id: light.kitchen
transition: 5
brightness: 255

There’s no mention of a dependency on brightness at https://www.home-assistant.io/integrations/light. Is this a bug, or expected behaviour? If it’s the latter I can file a docs update for this.