I've got the gut feeling we are not getting served the whole picture here about what's going on.
Never ever in 11 years of shooting with Blackmagic cameras (BMCC, 2 PCC4ks, 1 PCC6k Pro, UM46k, UMP G1) did I encounter a similar problem. All media formatted to exFAT.
The cameras itself are not capable of deleting anything - except when you format the media in camera.
In the other thread Ethan wrote, he first could playback the footage in camera, but after plugging the T5 to his Win PC, the footage was gone and then couldn't be played back in camera, too.
If the camera is behind this phenomenon then a firmware downgrade could be tested if it solves this.
I had the case, when I did the playback in one of my BMD cameras, then shut off the camera without going back into REC mode, Windows would see the media in need for a checkdisk repair. Didn't lose any footage, though. I think this was with the UM46k, but am not sure.
Then there could be an issue with Win 11 or that specific PC Ethan is using, or the cable/cardreader. Or there is some software interfering.
BTW - same thing can happen with an ARRI Alexa:
https://forum.arri.com/viewtopic.php?t=1589There is also an UDF issue with Win 10 and Win 11:
https://forum.arri.com/viewtopic.php?t=1464That got fixed by Microsoft 1 year later. Before that I couldn't read my Compact Flash cards from my Canon XF305 camera on my Win 10 PC (only my old Win 7 did read it).
• Windows 10: 2022-06 Cumulative Update KB5014666
• Windows 11: 2022-06 Cumulative Update KB5014668
It also depends on the negotiation between the reader hardware and Windows, which determines if a memory card is attached as removable media or external disk.
So I think if you want to find the cause you will need some kind of a systematic approach.