We have figured out what was happening. Python 3 has changed how DLLs are loaded on Windows. The GUI was taking of this correctly. We have modified code so that this bookkeeping is now down for both the GUI and when loading from python.
In the meantime here is a work-around. You have to add the ACIS library path to your environment PATH variable.
import os
import sys
os.environ["PATH"] += os.pathsep + "C:\\Program Files\\Coreform Cubit 2021.4\\bin\\acis\\code\\bin"
sys.path.append('C:\\Program Files\\Coreform Cubit 2021.4\\bin')
import cubit
cubit.init(['cubit', '-nojournal'])
cubit.cmd('brick x 10')
cubit.cmd('export step "junk.stp" overwrite')