Calendar events script API not working with Extended Attributes

I've been looking at Extended Attribute for some of the integration work we're doing and all was well until I got to Calendar Events. For some reason they don't behave as expected. Here's how I'd add an Extended Attribute to a Forum;

#set($options = "%{}")
$options.Add('_ExtendedAttributes_e14Matt5', 'hi')
#set($forumResponse = $core_v2_forum.Update(4, $options))

This works fine, and I see them in the response (running in the Script Sandbox);

When I do the same thing for Calendar Events it doesn't return anything;

#set($options = "%{}")
$options.Add('_ExtendedAttributes_e14Matt5', 'hi')
#set($eventResponse = $calendar_v1_events.Update(2, 1, $options))

When I look at the event via the REST API, there are no Attribs there either;

We're on 11.1.7, so I was expecting this to work based off other posts (here, and here) which indicate there might have been issues with Extended Attributes on this content type.

Am I doing something dumb here?