Allison trans manual lists temps of below 19f for a restriction of shifts.
I've never had the condition you describe , and I have started out in temps below 19f after a short warm up , but I had the block heater on, and went 1/4 mile through the RV park before I got fast enough for a shift.
Possible fluid temp sensor fault.
Do you have service records for the trans? Manual has a warning about cold operation with non approved fluids.
for a free download of the manual if you don't have one. Too big for me to post here.