Interesting. Found a GitHub issue that covers this bug: https://github.com/openzfs/zfs/issues/14290

The latest comment seems to be a nice summary of the root cause, with earlier in the thread pointing to ftruncate instead of fsync being a trigger:

>amotin

>I see. So ZFS tries to drop some data from pagecache, but there seems to be some dirty pages, which are held by ZFS till them either written into ZIL, or to disk at the end of TXG. And if those dirty page writes were asynchronous, it seems there is nothing that would nudge ZFS to actually do something about it earlier than zfs_txg_timeout. Somewhat similar problem was recently spotted on FreeBSD after #17445, which is why newer version of the code in #17533 does not keep references on asynchronously written pages.

Might be worth testing zfs_txg_timeout=1 or 0