Engine coolant temperature high - engine overheating

Hi, 

2017 1.6 TDI here. Manual gearbox. 

This first appeared 15 day ago. Coolant temp got high in a few seconds and fell down after turn off/turn on to 90. 

After that I noticed that sometimes temp would go from 90 nearly to red and returned immediately to 90. 

Went to service and conclusion was bad water pump because it was completely stuck. 

Changed that and timing belt, and the issue appeared again and now it was rapidly going to red altough the engine was not really overheating. There was no pressure in water tank and the water was not boiling. 

Oil temps were ok all the time. It is not losing water, not mixing water and oil. 

Do you have any ideas other than sensor. We'll change it tomorrow since it is not so expensive. I should've change it first but water pump was not always working so it seamed like the issue.