Autofs indirect mount is not unmounting

Hello,
I am attempting a simple configuration under Debian 12, autofs 5.1.18.

In /etc/auto.master.d/fortknox.autofs
/fortknox    /etc/auto.forknox   --timeout=20

In /etc/auto.fortknox
backups -rw,vers=4                 192.168.0.10:/backups

I can easily get it to automount with "ls /fortknox/backups"

But watching the mounts, it never unmounts.

lsof +D /fortknox/backups shows nothing.
lsof +D /fortknox shows that automount is accessing that path.

I have made no changes to /etc/autofs.conf or any other config file except browse_mode = yes.

I hope I am missing something simple, but after days of attempts I think I may have exhausted all configuration options. 

Is this a known issue that may be resolved in 5.1.19?

Thank you!
Don