null mounts seem to lose directories?

[nia <nia%NetBSD.org@localhost>]

I have various null mounts on top of a tmpfs:

$ df -h
...
tmpfs                                         87G   1.0G    86G   1% /sandbox/nb9-i386-trunk/chroot/1
/sandbox/nb9-i386-trunk/data/bulklog         237G    79G   158G  33% /sandbox/nb9-i386-trunk/chroot/1/data/bulklog
...

Directories are not being synchronized properly across the null mount:

procyon$ ls /sandbox/nb9-i386-trunk/chroot/1/data/bulklog/rust-1.52.1nb4/
build.log     checksum.log  configure.log depends.log   pre-clean.log work.log
procyon$ ls /sandbox/nb9-i386-trunk/data/bulklog/rust-1.52.1nb4/
ls: /sandbox/nb9-i386-trunk/data/bulklog/rust-1.52.1nb4/: No such file or directory

The source of the null mount is a ZFS dataset:

# zfs list
...
tank/sandbox/nb9-i386-trunk/data             124G   158G  79.2G  /sandbox/nb9-i386-trunk/data
...