ZFS property inheritance broken for `readonly`?

I've noticed recently (not sure when this started, maybe on the 12->13
transition, which would be the same as the old ZFS to OpenZFS
transition for us) that inheritance for the `readonly` property is
broken.

Here is an example:

[root@nfs-prod-11 /home/wollman]# zfs set readonly=on export/cl
[root@nfs-prod-11 /home/wollman]# zfs get readonly export/cl/u
NAME         PROPERTY  VALUE   SOURCE
export/cl/u  readonly  off     temporary

The child filesystem `export/cl/u` should have inherited the readonly
setting from `export/cl` but instead it (and all other children) have
overridden it, and I have to manually `zfs inherit readonly` on all of
the children to get the proper behavior.  This is quite surprising,
and the first time it happened I was sure that something had gone
wrong.

-GAWollman