Diesels run very cold anyway, so if driving at low loads with lots of heater its entirely possible for it to take an age to warm up. At idle a diesel will often cool down they produce so little heat.
The other issue is the temperature guage lies, and displays 90 as soon as the temps over 80c. Its entirely possible (if it was a derv) that you;d left it running for an age (wasting diesel and contaminating the engine internals with partially combusted fuel/soot) and it hadnt got to 90 at all, but was infact just on 80c, then when you cracked the heater open it dropped a few degrees and the temp appeared to drop from 90 to 70 when infact the actual engine temp only went from 80 to 75.
If its a petrol its certainly not normal, and i'd be looking at changing the stat pronto. If its a derv, i've no personal experience, but i'd still expect it should stay at 90 so long as your actually driving it, rather than just sitting idling.
If the stats never been changed, then i'd be doing that first, as contrary to popular belief its more likely to fail than a solid state electrical sensor.