Did you get this sorted? I’m having this exact same problem, but with Fibaro dimmer 2 modules. Would like to know how to use the transition function.
No, the problem persists, maybe the light-api from some cheap lights does not support the transition feature. 
I think I’m facing a similar issue overhere. The transition when powering on lights (ikea) is not working. But I’m also using a poweroff transition which is working very well
Only thing I’m doing different in poweroff is that the only property I’m providing is transition, and for poweron I also provide the property brightness and color.
I did not find any solution for that. And its the really tricky when i turn all lights off. Then some of them flash for a second but then do the transition…
I think its bc the lights are more or less “old” from an API-standpoint 
Could it be that the specific lights (brand) you use don’t support the transition function?
Had a similar problem. I used this instead for a slow wake-up transition and it seems to work. Maybe not as smooth as it could have been but not noticeable for my purpose node-red-contrib-light-transition (node) - Node-RED