It‘s been irritating when I experienced this in the first place, I know.

The idea behind:
TRS and XLR should carry the same level.
But this is not possible above +19 dBu, as TRS would run into clipping.

Why:

XLR’s electronically balanced out uses two active pins 2=(+) and 3=(–).

TRS’s “impedance balanced” out uses one active pin only, Tip (+) and passive Sleeve=(ground).
Ring does not carrying an active signal, but being connected to ground via an impedance matching resistor.

Therefore XLR is capable to deliver twice the voltage, or 6 dB more level, before clipping.

TRS can”t go above nominal 19 dBu (in fact 22 dBu) so when switching XLR to the highest Reference Level of 24 dBu TRS can not follow.
But for XLR’s continuous volume ramp, the DAC, used commonly for both outputs, is switched back by 5.5 dB at the 19 to 24 dBu transition.
This makes the TRS step down by those 5.5 dB at this last transition point.

Hope the above is understandable.

My simple workaround to get an unbalanced out with continous volume control:
I made a cable only using XLR pin 2 (+) and pin 1 (ground) an leave the TRS alone.

BTW: The DAC doesn’t need to have this behavior, RCA out simply is 6dB quieter in level.

I totally agree this behavior is irritating.

I don’t think it can be changed by firmware, as XLR and TRS aren’t fully independent in the hardware domain.

ADI-2 DAC’s 6 dB lower output on the RCA / unbalanced out vs the XLR is the better solution.
Even offers a good fit to the typical levels present in home-HIFI.