TW12: Audio input levels hitting a brick wall at -21dB ??

[bk]

I had this with a Clarett+ interface.
I had messed with the faders/settings in the mixer of the Clarett software and couldn't get full range of input anymore. The solution was to reset to default all the settings for my interface. Perhaps something like that has happened with your Behringer. On mine, it was a single mouse click in the interface software.
If you don't have a reset, make sure all the faders in your interface software are set to zero.