this is not realy a bug, it is just how it works.
there is still some space left on the vehicle and the vehicle does not know if the next thing that comes will still fit on the vehicle or not, it might be a very small car for example.
only when there is an other load available that is longer than the remaining gap, the vehicle will depart as full, since now it can be sure that it can no longer load anything else.

What an unfortunate algorithm. This really sucks because you usually want to make sure you don't leave the container loading station before you actually load something and if there is not a container ready the vehicle leaves without the "wait until loaded" command.

wait untill loaded is surely the better solution.

it is not to big of an issue, since usualy you will not pack only one container but do it continously, so there will always be a next container.
when the vehicle returns to load more, it will just pick up the leftover container and wait for more again.
so in reality you will only ever wait longer for the very first vehicle load, and will not be able to pick up the very last load when you stop playing the map. in the big picture this is no issue at all.

I just realized this could be solved by introducing a simple checkbox that would basically tell all the waiting vehicles in the container loading building that they should not expect any other container size than the one currently selected. This way a vehicle would know beforehand that there is no more room to load another container and could leave immediately.

It would be also great if we could automatically create containers the same way we build cars in the car factory. Meaning the container packing building would cycle through a predefined set of container sizes, maximizing utility of capacities of vehicles waiting to be loaded. Two medium containers + one small for example.