Gear shifting problem with open gas cap

Hi.
That's probably the title, but it fits.

Fiat Punto Selectra, also an automatic transmission.
Normally, the engine/transmission operates and shifts correctly and smoothly.
When opening the fuel tank cap, for example when refueling, there is a suction sound.
After refueling, the transmission only shifts to a higher gear at a significantly higher RPM, as if the gear lever is not in 'N' (Neutral) but in 'L' (Low).
After 500 meters, everything is fine again.
After a stop without opening the fuel cap, everything is fine, even overnight.
After opening the fuel cap, even without refueling, the same phenomenon occurs.
No, that's not my car.
Apparently, this has been the case for a very long time, or since the item was purchased used.

I'm currently interested in knowing if it's normal for the Fiat to have a vacuum in the fuel tank.
Furthermore, whether there's anything to worry about, or if you can simply accept it as a characteristic of an old car.

I need to thoroughly test everything again to verify the above.
It's not my car!

If I hadn't been there, I would have felt like I was being made fun of.
Best regards, Frank.

Hi.
The problem seems to be quite obscure, as no one is responding.
In the meantime, I've looked at the problem again, and it's actually the case.
After opening the fuel cap, the tank draws in air, and the automatic transmission shifts to lower gears much more aggressively.
At least the first 500 meters.
Best regards, Frank.

Then I would like to find out more about the gearbox's operation. Is it an automated manual transmission, and does it use oil pressure or pneumatics for shifting?

"Is there a vacuum sound coming from the fuel tank? It might be an overly aggressive fuel tank ventilation system that's directing dissolved fuel towards the engine compartment and an activated carbon filter..."

Is it a gasoline or diesel engine?

If your activated carbon filter isn't working correctly, it will create a vacuum in the tank. This prevents the fuel pump from working properly, which results in a loss of power. If there's a lack of power, the engine might not reach its full RPM. This would be similar to running at partial load.