Is there a known issue with Event Update Record Action dropping related contacts in Lightning?

I have Update Record action on Events with the basic fields:

Start, End, Type, Event Status, Subject, Location, Description, Name (Contacts).

I can add additional contacts to the Contacts field through this action button in Lightning.  And the primary contact has the +1 next to it, and the related contacts are in the related view.

However, when I open this action to make an update to the event, only the primary contact is listed in the contact names, and after I save, only the primary contact is left related to the event. 

This only appears to happen in the browser using lightning.  When I tested this using the mobile app, even thought the update action only shows the primary contact when I'm on the edit layout, after I save the updates, the related contact is still related in the mobile app.