YeeLight 1s bulb (YLDP13YL) disconnects without wan

Dear YeeLight team, I am facing some issues with my YeeLight 1S bulb (YLDP13YL). According to the product descriptions and forum posts it should be possible to use YeeLight bulbs on a local network only without an internet connection as described in the following links.

YeeLight inter operation spec


When I use this method with my 1S bulb. the bulb will stop working after a short period of time (about 15 minutes).

I have had this problem for a while now but it changed over time. On firmware 2.0.6_0028 the bulb rebooted every 15 minutes where it stopped working for about 40 seconds every 15 minutes. After facing this problem I decided to update the bulb to firmware 2.0.6_0031 this firmware makes the bulb unusable after 15 minutes (does not reboot by itself anymore) after cutting the power and repowering it it works for 15 minutes again.

Because the behaviour of the problem changed heavily with software updates I think this is a software problem.

I have done several tests with telnet, HomeKit and checking my access point connections. all connections drop simultaneously.
when the device has an internet connection device will not disconnect.

I hope you can help me to solve this problem.