This is an FYI, hope someone finds it useful.
It came as a surprise to me but perhaps I am doing something unusual.
Adding it to ORDS Category because encountered error during ORDS install - but error is really on LB.
Installed ORDS 25.2 on serverB and all seemed well but got 502 error.
Root cause was Health Check on LB for certbot on serverC failed ( which I use/need infrequently).
So fixed that, and 502 goes away.
It is un-intuitive to me (a non-network person) that a Health Check on ServerC should cause error reaching ServerB.