I received more info from Tanki Online about your situation. Here is what I got:
I have some updates regarding the issue with user 4MATIC. Please find my comments in lines below:
1 . What happens for cross-sites accounts? If someone have account on your site and wants to transfer to ours, or vice versa?
When the user registers on partner's websites, he does not receive the password which he could use to enter the game on our website. And unfortunately, we do not have any migration system to transfer the users from partner's website to our website or vice versa.
2 . Does these events he cannot join, are exclusively limited for Tanki Online the original site? Or its working on our partnership version as well?
Most of our events are equal for all our users. There are only a few of them that could be available only on the main website. It would be great to find out which of the events the user 4MATIC could not enter.
3 . Is there any difference in the amount of players between our versions? I mean, just an example: Tanki on your site has 10k online players while on our have 200 online players because they are the ones using the Y8/ID login which makes them aside of the Tanki crowd?
All our users (who came from partner's website or registered on our website directly) play all together on the same servers. It does not matter where they come from.
So lets discuss about this, BING, which events you couldn't join from Y8 website? Maybe there was some internal bug on their side, so I will update them on that.
As for transfer, it seems it might not be easy for achieving that without messing the database between the overall partnership.