Reflowed the DAC and now it's definitely improved.
IN2/3 is flatlined despite having valid input and Vref looking correct.
Probably a solder defect, these are ones that I reworked early on and it looked good visually but I didn't do a functionality check yet as I didn't have the necessary firmware done.
Also definitely need to debug why the firmware suddenly stops responding when I'm sending it SCPI commands, then is fine when I reconnect.