@Blacky I did some more testing and I think I finally found what’s causing the issue, even though I don’t know why it’s causing the issue. It was not about an ON or OFF signal, but for some reason, if the option ‘Use Transition’ is on, the bulbs that were off will turn on. I got suspicious because I noticed that the OFF bulbs actually turned on a fraction of a second before the ON bulb turned off.
If I remove this ‘use_transition’ option from the YAML or just disable it in the UI, the automation works as expected. I guess this is a problem with the Ikea bulbs. I have some old WiFi bulbs in a drawer somewhere, too, so if I find an opportunity I’ll test them as well to see if it’s maybe a problem in Home Assistant itself. Either way, the transition is not that important because the lights are on very low brightness anyway, so I will just leave it disabled and enjoy the automation working perfectly. 