I tried it on two different host distributions (gentoo 4.14.143-gentoo and archlinux 5.4.25-2-lts). The sysctl
/proc/sys/net/core/rmem_max was not available in the containers of these machines.
Do you have /proc/sys/net/core/rmem_max in the containers of the Ubuntu host?
The naming core may suggest global kernel limits though, so those may not be namespace-able. Probably still worth asking the kernel folks about those though.