Incidently I have come up with a workaround if anyone else is having this trouble. Somewhat complex but acceptable as this feature was a major part of my smart home routines. I use the Smartthings app and an alternative smart plug. I used the plug my kettle is connected to. It doesn't matter if the plug is on, it just means my kettle will just have power if I need it. You could use any smart plug that doesn't have energy use consequences if turned on.
I set a Kasa Smart Action to turn on the smart plug that my kettle is plugged into. I set the 'On Duration' for just one minute.
I set a Smartthings routine to turn on the kitchen light when my 'Kitchen Kettle' device is turned on.
I set a Smartthings routine to automatically turn off the kitchen light after 20 minutes of being on.
I set a Smartthings routine to turn off the kitchen light again if it gets 'turned off', with a delay of 30 seconds. (If device is turned off, then 'turn off' after 30 seconds).
So this is what happens:
I walk into the kitchen, motion is detected and the kettle comes on.
Smartthings automatically turns on my kitchen light, because the kettle has been turned on.
20 minutes expire.
The kitchen light goes off.
Smartthings is then triggered to turn off the light again in 30 seconds.
The Kasa smart action is triggered again by the light turning off, It turns the kettle back on and Smartthings turns on the kitchen light again.
30 seconds later, Smartthings turns off my kitchen light again, thus ending the infinite on/off light scenario.
If I am actually in the kitchen when the light goes off and it was me that triggered the smart action again, I only have to wait a minute for it to reset and turn on the kettle again and for Smartthings to turn the light back on. Or i can just turn on the kettle plug manually.
if I was to just use the Kasa app smart action to turn on the light for a duration of 20 minutes, and use Smartthings to turn it off again after being retriggered, I would then have a 20 minute wait until the smart action 'On Duration' ends, hence using the kettle method with just one minute 'On Duration'.
It's a little annoying to have the kettle smart plug keep clicking on every minute if I am in the room but this is an acceptable workaround until/if the Kasa smart actions for motion detection are sorted out. It was totally fine before the recent big update.