When using the mass update form "Update Virtual Servers" to update all servers, several other things were changed that should not have been.
I selected only one item on the form:
DNS domain enabled? -- set to Disable
All other items were set to "Don't Change".
On the following status page, most domains simply had two actions, as expected:
Deleting DNS zone ..
.. done
Saving server details ..
.. done
However, some domains had these additional actions:
Modifying administration user ..
.. done
Updating Webmin user ..
.. done
Updating owner limits ..
.. done
Afterwards, I noticed that these domains had their bandwidth limits reset to unlimited. Also, when reviewing the Webmin Actions Log, it appears that some password hashes were changed in miniserv.users and /etc/shadow, and some quotas were modified, as in:
Executed command
edquota -u xxxx
Most of these changes appear to be minor, but I will apparently have to reset the bandwidth limits manually back to their former values, and I'm not sure why or how the password hashes and quotas were modified.