When I try to save custom data in admin edit (album or image) I get an empty field, unless filter-custom_data is enabled.. Then the field is called Special data and it works. I didn't find the documentation for this filter plugin, that is a new one afaik.
Are you planning to disable the custom data option without the pulgin enabled?

Seems to be a bug. We will figure it out.

Fix will be in tonight's nightly build.

Great!
Can I have some info on the usage of this filter plugin?

The filter is intended only as an example to start from if you wish to create your own handling of custom data. You can review the plugin architecture document on the zenphoto site pages for general details and look at the plugin code for specifics of what it does.

With such a plugin you could, for instance, recast the custom data field as multiple fields or do special verification of the data input.