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

Configuration
1

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.