The SPG-15 mission for the Stug IV is bugged:
Thank you for your bug report!
Please be reassured that this issue is known to us and is currently being fixed by our developers.
Please keep an eye on our patch notes to see if this bug is corrected in a specific patch.
We shall contact you for additional information, if needed, and we sincerely appreciate your interest in the improvement of World Of Tanks. We unfortunately cannot mark this mission as complete, but we will be working to make sure the bug is patched out.
Please do not hesitate to contact us if you have any further questions or concerns.”
So I guess that’s that, be aware that at least this mission, if not others, are known to be bugged by WG and there’s nothing to be done about it until it’s patched, even if you have proof that you’ve completed the mission.”
(souce: redditor Dauss)