It is supposed to be like that. I personally wrote a ticket about this and got it confirmed. It is intended and exactly working the way it shall work.
Here in short the explanation I got:
"Online Duration" is a term made up by the users, it is actually called duration. Look closely at your own shop. You will only find the terms "duration" and "offline duration".
With "duration" is meant the timer (that is counting down) that tells you the time you have left to use the premium/special shop features, like more slots and the offline duration. The feature "offline duration" enables the shop to be open when you are offline WHILE the premium/special shop timer is still running.
In short:
You rent a premium/special shop for a certain time. While renting of this is ongoing you are allowed to have a second timerthat only counts the shop time while being offline.
As soon as the overall duration that enables you to use these features is over, the offline time stops to count down and the market is closed while being offline.
The ticket answer said that the offline time is then being "accumulated" and activates itself again as soon as you activate a new shop of the same type special/premium. But if you decided to downgrade the shop to a basic one, the offline time you had left vanishes with this step.