I was having this exact same problem but on windows. I eventually figured out a workaround but it's a real pain.
What I did was sign into Steam in a browser and navigate to the Deadfire store page.
I then had to navigate to each individual Free DLC (I didn't get that infinite block in the browser) and hit download. I selected the I already have Steam installed option and the Game auto-launched after hitting download on each DLC.
Once I quit out of the game the DLC showed up in the DLC section for Deadfire and I was able to toggle the check mark box and I was able to start downloading.
Not Sure if you're still having the problem but even if you aren't I hope this will help future peoples since I haven't found any kind of help for this particular bug.
