In what world is this normal? Have any of you ever driven a tdi? I have, I owned a mk4 diesel which was so efficient at retaining heat that my coolant would take a long time to get to temp when it was very cold. When it did, it would only go down rapidly if I had the heat on high while i was idling, but while driving, especially on the highway at higher rpm, it wouldn't go down... it had to be extremely cold, and even then it would maintain coolant around 80-85 degrees Celsius (not the optimal temp of around 90 but still)
This is a gas engine, there is absolutely zero reason for the coolant temp to be dropping so drastically, especially in the scenario and temperature that the OP is describing
OP, just do what I mentioned and report back. If your coolant is indeed dropping in temp so drastically then I'll stand corrected, and i'll also be really surprised and confused as to what could be going on