When changing light bulbs (KL125 in my case) between two states, the transition feature works as expected when changing the color or changing the brightness, but not both. I'm not sure if this is a limitation of the bulb or a bug in python-kasa but I wasn't able to find this behavior documented anywhere.

To reproduce from a command line, in order:

Set bulb to dim blue; Works as expected
kasa --type bulb --host 192.168.9.11 hsv 240 100 1

Transition to bright blue over 5 seconds; Works as expected
kasa --type bulb --host 192.168.9.11 hsv 240 100 100 --transition 5000

Transition to bright red over 5 seconds; Works as expected
kasa --type bulb --host 192.168.9.11 hsv 0 100 100 --transition 5000

Transition to dim blue over 5 seconds; Does NOT work as expected. Fades to dim pink/red and then changes to blue at final moment
kasa --type bulb --host 192.168.9.11 hsv 240 100 1 --transition 5000

Transition to bright red over 5 seconds; Does NOT work as expected. Changes color immediately and then increases brightness
kasa --type bulb --host 192.168.9.11 hsv 0 100 100 --transition 5000