Issues combining CF Load balancer with external load balancers

I'm trying to combine the Cloudflare Load balancer with regional load balancers at Hetzner and Digital Ocean, but when adding the public IPs as endpoints in Cloudflare, traffic just stops working.

When using for example the public IPs from one of the droplets behind the load balancer from Digital Ocean, then there aren't any issues.

What's causing this? Am I missing something?

Thanks.