Because the battery is too cold to accept regen charge, don't ask me to explain the science
I am sure someone else has a dissertation on prior threads.
To combat limited regen I use scheduled charging for 5:30 am and set the cap to 80%. Each morning when I leave home around 7 am the battery is warmed up and ready to go with full regen. However, sitting outside at work the battery will get cold soaked and regen is limited, how limited depends on how cold it is and how long it sits idle. It was 29 degrees in my garage this morning and all was well. At 22 degrees most of the day the battery got cold enough that regen was limited for my entire (short) commute of 7 miles.
@OP, I concur that your regen limits are due to ambient/battery temps.