Vent leak
Yes and just being a smaller area to work in there is less area for the pressure being built up at cold temperatures also. Try the rubber hose length trick....that should solve it as long as you go up with it. Or you might need another vent hole OR larger vent diameter hole.
When the oil warms it tends to become more like a liquid instead of a syrup when cold. When it is thick it could be clogging the vent and pressure builds then blows through when too much pressure builds behind the thick oil. Then when the fluid thins out from the heat this trouble stops because the oil drips off faster.
See how it goes. Jim