I now made a DIRIGERA my only TBR and switched off the HA OTBR. To debug and check what is happening on my Thread / Matter network, I created a bit of an AI slop app so I could figure out what is linking to what, but hey, it helped me:
Just clone and open index.html in chrome. Interesting detail: The json coming from the matter server is sending the neighbour node addresses as INTs, so its rounding them to json precision and they are not correct anymore, so this is now just using the first 48 bits for matching nodes. YMMV.