Field not consistently read-only with inline editing

I've got a custom field and I've created a behaviour that should make that field read-only ... except that it doesn't. I can still edit the field. I've tried turning on logging for Behaviours but I cannot find any logging output for the plugin.

Here are the settings I've used:

Use Validator Plugin: Disabled

Guide Workflow: None

Field: Organisation, Optional, Readonly, Shown, Conditions: None.

Mapping: Developer Technical Support (the project), all issue types

Here is the XML:

<config use-validator-plugin="false" name="Read-only organisation" description="Used to make the Organisation field read-only" guideWorkflow="null">
<field id="customfield_10601" required="null" readonly="true" hidden="null"/>
</config>

This is affecting the inline editing capability in JIRA, in that hovering the mouse over the custom field causes the editing UI to be displayed. HOWEVER, if I then click on the Edit button then cancel, the inline editing is correctly disabled. Refreshing the page causes inline editing to work again