Bump but with an update, I discovered something strange that got it to work without having to do the temporary fix. This was found with the 8bitdo Ultimate 2 Wireless in 2.4ghz Dinput mode (Home + B):
I connected it to Steam before starting Deadlock which would normally break it from working but before launching the game I opened the “Calibration & Advanced Settings” window, closed it, opened it again, closed again then launched Deadlock with the controller still connected and finally the game detected inputs.
My theory if this can be replicated consistently is that Steam is not recognising the controller for SIAPI supported games (these are games with official Game Action controls) and bringing up the settings twice sort of jolts the software to become aware of the controller and use it properly. This could be completely fabricated but I have no idea why this solution also works.
This is probably an issue that’s deeper than Deadlock, luckily the developers work for the same company so it shouldn’t be a big ask to get a Steam Input developer to check this out.