"Oil level is actually low"
That plus a clogged filter might be why.
If the filter is clogged (return hose jumps), the oil is probably unseating the filter bypass relief valve.
The relieved oil might be shooting out of the bypass valve at an angle that directs it by chance to the vent elbow pipe in the tank cover.
With oil hitting that elbow while the tank is venting out, oil will be carried out too.