Behaviours ignores multi user picker custom field

Steps to reproduce

0. Create a Multi User Picker custom field with name Multi User Picker
1. Set up a behaviour for field Description
2. Select writable
3. Add a condition: except Current user in user custom field value Multi User Picker

Result The description field remains writable even if the current user is part of the Multi User Picker field.

Workaround

Instead of setting the condition from the UI, script it. For example

def currentUser = ComponentAccessor.getJiraAuthenticationContext().getLoggedInUser()
def description = getFieldById(getFieldChanged())

def cf = ComponentAccessor.getCustomFieldManager().getCustomFieldObjectByName("Multi User Picker")
if (currentUser in underlyingIssue.getCustomFieldValue(cf)) {
    description.setReadOnly(true)
} else {
    description.setReadOnly(false)
}

For more examples on how to create scripted conditions