Additional attributes do not get saved

Currently it seems like custom attribute fields don’t seem to get saved to database correctly when using fronend submission.
When monitoring the submitted form data via dev tools you can see that everything gets submitted correctly (organizer attribute in this case):

event[event_name]:title
event[event_notes]:description
event_attributes[organizer]:example organizer
event[event_contactperson_email_body]:
event[event_url]:
event[event_start_date]:2016-02-25
event[localised-start-date]:2016-02-25
event[event_start_time]:17:00
event[event_end_date]:
event[localised-end-date]:
event[event_end_time]:
event[location_name]:
event[location_address]:
event[location_town]:
event[location_latitude]:
event[location_longitude]:
event[action]:new_event
new-event:4706df39cf
_wp_http_referer:/veranstaltungen/termin-einreichen/

Is this a known bug? Any idea why this happens and how i can fix it?

https://ww.wp.xz.cn/plugins/events-made-easy-frontend-submit/