It is rather unclear what is happening. So I went through the entire (lengthy...) source code of the network table processing and ensured error handling works in every case. The code has grown over half a decade and could very well take some unification and slight simplifications in a few places.
In case you are not using Pi-hole v6 docker but have a native install, please run
pihole checkout ftl tweak/network-table-error-handling
and see if this improves the error situation. Even if it doesn't (likeliness is maybe 20%), the improved error handling should hopefully give us more details about what is exactly going wrong here.
In case you are using Pi-hole v6 docker, you'd need to build a customized image yourself, see README.md/Building the image locally. Your command would be:
./build.sh -f tweak/network-table-error-handling
resulting in a new image you can thereafter use like image: pihole:local in your docker compose file.
Still somewhat grasping at straws as to what the exact cause is - but we will get there! Thank you for your help!