There is also a procedure in the handbook to allow the adblue level sensor to update after you re-fill. You need to turn the ignition on for ~30 seconds without starting the engine. The display normally updates and resets after about 10 - 15 seconds then on mine. After that, start engine and carry on as normal.
I certainly wouldn’t store an open bottle of adblue. It horrible stuff and goes crystalline. It certainly has a limited shelf life. I buy the VAG branded stuff from a dealer & keep all the receipts. I know it’s more expensive than using a forecourt pump, but if there are any future problems with the adblue system, then they can’t use the "contaminated adblue" excuses. Adblue system repairs are VERY expensive!