Looks to me like the original implementation of `Bitfield` back in 2018, that was first used with the mec applet, simply ignored incorrect field names. I imported that old code into python, and ran the following:
```python
>>> AUX_STATUS32(halted=1)
<__main__.AUX_STATUS32 H=0 E1=0 E2=0 A1=0 A2=0 AE=0 DE=0 U=0 L=0>
>>> AUX_STATUS32(H=1)
<__main__.AUX_STATUS32 H=1 E1=0 E2=0 A1=0 A2=0 AE=0 DE=0 U=0 L=0>
```