Yeelight Color - Lower color/brightnes after a 15-20 minutes

I have plenty of Yeelight RGBW leds, and I still use them plain, without anything around it, just hanging from the roof with an E27 socket+wire to achieve the longest lifetime.

I’ve noticed most of my active yeelight bulbs drastically lower the brightness and change color temperature after 15-20 minutes of use. In Yeelight/Mi home App nothing changes, but physically it does change, kinda like 100%->70%.

Air conditioner is ON, the room temperature is about 25°C. They are from gearbest/banggood, most of them around 5-6 months old, with the latest (1 week old?) firmware, I usually bought 1-3 at the same time in 1-2 weeks differences.

Am I alone with this issue, or Yeelight RGBW leds have only half year lifetime?