Trying something really stupid, I seem to have solved it. This seems highly possibly to be a bug...
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pi_dev_server2 is the pre-existing server backend, just renamed, and it uses the hostname as mentioned in my first post, which gets correctly resolved to 10.0.0.235 by HAProxy and uses port 80. There's no obvious reason why this doesn't work, but it doesn't, I've disabled it here for the test. But if you look at the original post, it fails with L4OUT.
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Now all of the servers are working, as a result of that one change (?) or did something else suddenly start working? I'm not sure, but having pi_dev_server using an IP instead of a hostname seemed to make all of the others work properly, despite the fact that they are still using hostnames... Very very curious...
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