Apparently my repo list is a liar. After purging 8, 11, and 12 from my system, installing 8 again....
$ apt list --installed openjdk-8-jre
> ... [installed]
$ /etc/alternatives/java -version
> ..."11.0.4"...
$ java -version
> ..."11.0.4"...
I'll give 8u221 from oracle a shot.
On a side note, Minecraft does work without Forge.
I guess it's because the binaries are being compiled differently?