Zombies without a parent process occur when running without an init process that reaps the orphaned child processes. Normally, this shouldn't occur, because orphaned children are adopted by init (PID 1) which reaps those orphans on termination. However, there are two scenarios where this can happen:

• Inside of a Docker container: It is not usual that containers are started without an init process. This is also the place where I stumbled across this problem.
• rescue mode: Instead of running init on boot, you boot into a shell in order to repair a broken system. Actually I haven't verified this part, but it's likely affected, too.

Note that I'm guessing what's causing the differences between the ps auxf output and that of htop, but I'd say it's a reasonable explanation of what I can observe.