I investigated that topic a little further and like to share my findings, so that it may help someone in case running into a similar issue in future.
First I was wondering if there is something special in my setup which differs from usual environments, and one thing what me came in mind was that my specific HW (Fritzbox 7312) do has only one single eth port (no further switch integrated into the device). However, I do not know if this contributing or even causing the issue finally.
After spending some time to look for similar issues, my consolidated outcome is the following:
udhcp is not responding to plug-events of network cable and that this may also do not make much sense in general since in typical case of switches behind physical ports such events would not appear.
https://dev.archive.openwrt.org/ticket/9367
However, use of netplug or ifplugd has been proposed as potential solutions.
Furthermore and even simpler to install, it has been recommended to use dhcpcd (which monitors link status out of the box) instead of udhcp by installing the available package.
https://lists.alpinelinux.org/~alpine/devel/<7edd13a1-bb39-60d7-c092-7694796260f0%40ustc.edu>
But accordant to https://unix.stackexchange.com/questions/4014/how-do-i-use-dhcpcd-in-openwrt-instead-of-udhcpc , the installation via
# opkg update
# opkg install dhcpcd
will not lead to a well working system since udhcpc is hardcorded into the helper scripts of ifup on OpenWRT and after installation of dhcpcd both programs gets executed together.
Finally, the issue could also be caused by some unnecessary but failing checks in dnsmasq init script, which results in dnsmasq not offering dhcp v4 adresses, with a simple workaround to set the "force" option to the "lan" section of /etc/config/dhcp and which is also confirmed to be used by the original firmware used by GL.iNet.
Eventhough all that feels for me like a bug in a release for a device with only a single eth port and I would be happy if this gets fixed in some future release for the Fritzbox7312, I am now able to handle it and do have some workarounds available.
With that i will close this issue. Whish you all fun with OpenWRT on your devices.