I'm working with a client org that uses the Field Service Lightning managed package. FSL includes a few fieldsets we can edit in order to make sure certain fields show on certain pages.
The FSL Service object has one such fieldset called Service Lightbox. When I go to edit the fieldset, I see a whole bunch of my custom fields and many of the fields that came with FSL. I am able to add fields pointing to related objects such as Account, Asset and Resource. However for whatever reason the related Contact does not appear as an option. For a different FSL fieldset, Services List Columns, there is no such issue and I'm indeed able to add Contact to the fieldset. Similarly if I create my own fieldset I can add Contact to it.
Is there anything I'm missing here? What drives the available fields I can choose to add to a fieldset? It really is essential to add Contact here and it does not make sense that it's unavailable.