Thanks for your input Andrea, I checked and all routes and locations are set with correct push rules. I did some more testing and I think I have figured out what is happening and fixed it.

For the original warehouse A I used the default "receipt in 1 step" route and modified it to our needs. However when I configured the second warehouse B I archived this default receipt route and created my own route. Both routes are configured identically, but I think the rental app platform is only set to trigger the default routes, not any newly made route. So for warehouse B I just deleted my custom route, unarchived the default route and edited it to be the same as the custom route and now everything works and the return operations are triggered successfully. So despite the two routes looking identical, it made a difference whether I made the route or Odoo produced the route as a default. I couldn't find any documentation on this behaviour so hope this might be helpful.