Ok so this is a bit hackey, but after trying several things the ultimate “fix” was to just use balenalib/raspberrypi3-debian-openjdk:8-jdk-stretch-run .
There may be some way to force 32bit Java 8 onto aarch64 but I think it’s easier to do it this way.
Oddly enough, Raspbian built for the Pi4 does use an arm7 build of Debian.