Assuming it's a bug, you can report it
here on the issue tracker.
If you want to try and get reimbursed for the loss of mission credits (again, that's assuming it is a bug), you can
submit a ticket here. (Probably not worth it to be honest, they seem to be busy and it's taking 72 hours to respond to a ticket.)
From what you've written, there's no reason that I can see why the depot won't accept your mission cargo. You loaded it from the originating depot as you say it's the mission specific cargo (should be labeled as haulage I think), meaning you didn't buy it, because that doesn't work.
You are at the correct destination, and the depot there is expecting two tons of Bertrandite - do you have any other delivery missions, and is this depot expecting something different?
As mentioned above, you have first clicked on deliver cargo before trying to click on the + button to deliver it.
I've done hundreds and hundreds of delivery missions, never yet had a problem unless it was my mistake.

Not saying it cannot happen, so if you wish to report it, you can use the links above. They will ask for more info if they require it.