And there we go.
In this apache installation, I was overriding (what I thought was only) the 502 error. However
ProxyErrorOverride On
ErrorDocument 502 “The backend server is down.”
I thought this would only override http error 502, but it overrides everything.
According to the apache docs, the following config should allow only error 502 to be overridden
ProxyErrorOverride On 502
but unfortunately, it still captures the 423 return code.
I’ve disabled this for now until I figure out how to configure apache properly to achieve what I want without breaking the mod_proxy / tomcat9 configuration.
tl;dr; Don’t override apache’s built in http error handling when proxying to xwiki/tomcat or some things will break