*I'm not an English speaker, so I'm posting using machine translation.
*Please understand that the text may be incomprehensible.

I had the exact same problem when using a DS4 controller, but I was able to solve it later.

In my environment, I was able to solve the problem by doing the following:

(1) Open the Steam Input controller settings.

(2) By default, the trigger item is assigned to only full pull, but set the exact same action assigned to full pull to the soft pull as well.

I was able to get the R2 trigger to work using the method above.

However, it's been quite a while since you posted, so the problem may have already been solved. If that's the case, I apologize.

Try turning off steam controller support. The game already has it built in and causes conflict issues.