Ugh - I feel so sorry for the folks at Garmin.
They have been really good to work with and very supportive of Subsurface. I was disappointed that they were adamant that Subsurface shouldn't try to connect with the Descent via BLE - but given how supportive they were in other ways, I didn't want to ignore their very strong request.
So unfortunately you need a PC/Laptop to get data off of a Descent MkI, but at least that always works, regardless of there being hiccups elsewhere in the internet.
Yes. That is totally asinine and one of the biggest Cons to buying a Descent (to me). Both that I can't download from it via BT, and the reason behind it (because Garmin wouldn't allow it).
I usually download from my Shearwater to Subsurface-mobile (via BT, of course) on my phone during the boat ride back from diving. SO convenient to get home, open Subsurface, and all my dive data is just there, ready to review and add notes.