is it normal for tanker trucks to sit in an open left lane and move at the same speed as right lane traffic???

for context, I’m driving on I5 north where there’s a big closure happening because of the landslide, and several times I have gotten to the point where there’s a detour off the highway and there is just a tanker truck sitting in the left lane going at the same speed as right lane traffic despite the fact that the entire left lane up until the left lane ends is open. Is there a reason for this, or are they just being dickheads?