Since the arrival of 1.2, there's a small bug in the key preset system, when you set it to "Legacy" to keep the pre-1.2 keys. This creates a bug with the RB key, especially when aiming.

You simply can't use the shot selector, grenade launcher or bipod in ADS mode.
Note: this only works if you modify the key with the new key, otherwise it won't work.
Note 2: It's not a mod that's creating this bug, it's really since the introduction of the new buttons for PlayStation
Thank you for taking this bug into consideration as it is truly annoying.