A puff of White Smoke

My Van 2016 Citroen Relay non AddBlue, will put out a puff of white smoke just for a second when I'm changing gear or accelerating. But not all the time only every now and then. It lasted for 5 minutes and then it stops. The van has plenty of torque and power. I have the van since 2019 and has always been happening. Is this the engine regen? Any ideas.

It will not do when there is no load on the engine by just revving the engine by it's self. it only happens when the engine is under load e.g. putting the foot down when driving along the road or just after you change up a gear and accelerate again for the first second there is a puff of smoke and then all clean again until the next gear change. Coolant is fine. She would use a bit of oil over the year say from the top level of the dip stick to the half mark on the dip stick, but never have to add any oil between the yearly oil change. There is 89000 miles on the clock I'm going to get the belt changed before the summer as she is 8 years old so just in case.

I have done a bit more research on the white smoke. I've read that the thermostat could be stuck open, so when the engine goes into regen mode the temperature in the engine is not hot enough, so it's not burning the carbon properly. I have noticed that the van will take a long time for the temperature gauge to start to rise when I start to drive it.