If the tank percentage fell quickly after a delivery, especially if the weather has been cold, this is normal.
Although your tank gauge may read 80% right after a fill, inspecting the gauge the following morning may show a percentage drop of up to 5%, even if no gas has been used.
This occurs because your propane gas is delivered as a liquid. Like all liquids, it expands as temperatures rise and contracts when temperatures fall. (Actually, the liquid propane in a tank will expand 270 times its volume once it vaporizes into a gas.).
You will still have the same number of litres of propane, but it will occupy less (or more) volume depending on changes in temperature.
For this reason - whether the tank is filled partially or fully - the delivery driver relies on the bleeder valve (and not the gauge) to measure how much propane fuel is being added.
Important reminder: The gauge reading on your tank only measures the level of liquid remaining in the tank. It will vaporize into gas, which is what your equipment and appliances burn in order to operate.