@Mizogin (7 February, 2017, 5:00 PM UTC):
A possibility is that you are accepting them as they are earned, or attempting to after they expire. Unlike Tournaments, you receive your clan mission rewards as soon as you earn them, not when the event is finished, so you may be accepting them before you can get your official report. Another possibility is that you are attempting to accept them more than 3 days after they have been earned (Again, 3 days after the threshold has been achieved).

What I would do is next time (Before accepting any clan mission rewards), take a screenshot of your current inventory, take a screenshot of what you should have earned (Reported after the event is done), and then another after you're screenshot the report; Include these in your Help and Support Desk ticket under In-game losses (If you haven't chosen that as your ticket subject).

I do recall something like this happening a long while ago, where (I believe) there was a legitimate bug where players didn't receive their appropriate rewards; However, this occurred to many more players (Unlike the current appearance), making me think that this is more specific to you.

Mizogin said:

I've noticed that almost always I receive only 1 reward pack from clan events while I should have received 3. Or I don't get them at all. Sometimes they appear in the Black market after I reload the game, but most of the time they don't. It's very strange and uncomfortable. Have you noticed something similar?
I've contaced the support, but they said that it's a game mechanics question and closed the ticket.

Yes.. It is a problem here and on Stormfall, they eventually come in but at random times. I once left my clan then quit the game and when I returned 2 weeks later I had 31 expired rewards in the BM. I had 0 rewards unclaimed/expired when I was logged in previously.