In a view for one of my group, some of the new tickets are being shown as 'new' and assigned to ' - '.

While some are being assigned to the default group 'Support' and set to 'open'.
There doesn't show to be any previous owners so I can't figure out why these in particular are doing that.

Same view, same ticket type and tags and form. Different statuses.
You're correct trigger and automation makes an update to a ticket like assigning it to a group or an agent which changes the ticket status to open.
Sincerely,