Nothing changes immediately. Measurements are taken every 15 minutes, so you have a short window to put the cylinder back on before the next measurement cycle.
However, if the Senso4s device is empty during the next measurement, it will record a value of 0 kg. Putting the bottle back on after this measurement will not restore the correct reading automatically; the device only logs changes if the new weight decreases by at least 200 g compared to the current measurement, meaning it ignores sudden large increases.
To get everything working accurately again, you will need to perform a new setup in the app. Please note that during this repeated setup, the zero-adjustment step can be skipped, and gas cylinder does not need to be removed from Senso4s device during the process.