Basically what you said, they don't want half the roster to be Russian and they'd rather sign Chinese fighters.
Valiev had to wait two years to get in pretty much, they still haven't re-signed Tumenov. Magomedov got bored of waiting and signed for Bellator, same with Kerimov and Vakhaev returning to ACA.
Mineev could be a decent challenge to Izzy and they haven't signed him.
It's shocking really. Thankfully there's a decent amount booked on the contender series this year.
Mineev few years ago was like -- people are thinking that UFC is a good promotion because Russians thinks that if something is American it means this thing is good but in reality UFC and their fighters are not better than Fight Nights/Russian promotions, I can find good opponents here and not gonna sign with UFC. Then he was talking about fighting at WW in mma, in reality he fought a few times at some catchweight and never wasable to make WW limit. Recently, I think he became kinda warmer to UFC. But -- he is friends with one of Fight Nights founders and he is getting paid millions for fight with Ismailov, if I'm not mistaken. I also remember he was talking about fighting A. Emelianenko under boxing rules this year. I think he also said that he was planning to fight 2 times before ismailov fight, one of the fights as far as I remember was supposed to be a kickboxing bout versus K. Gluhov (seems it is rescheduled or something, because of COVID). So I think he is getting paid some good money in Russia and I doubt UFC is interested in wasting money on him or allowing him to have boxing and kickboxing bouts.
Like I get that they don't want to have too many Eastern European in the UFC since their main audience is America and Easterners don't sell, but why don't they just release the bad ones they do have?
They were quick to sign someone like Khadis Ibragimov or keep guys like Roman Kopylov on the roster but pass on guys like Rustam Kerimov, Mukhamad Vakhaev, Albert Tumenov, Mateusz Rębecki. Even Mateusz Gamrot vacated his belts in 2019 and was a free agent, but eventually signed a two fight deal before UFC finally budged.
Not even Brazilians have this much trouble getting signed and I would say they usually aren't draws as well.
About guys like Ibragimov -- he was M-1 champ and they had agreement that M-1 champs can go to UFC Kopylov was Fight Nights champ (am I right?) so it's not like they signed some nobodies.
I am pretty sure that Kerimov, Vakhaev and Tumenov had problems with American visa so there is no reason in signing them during COVID times when like 90 % of UFC events happens in US. Plus there is already signed multiple Russians with visa problems -- Emeev, Taisumov, Gamzatov, Fakhretdinov (it's on the top of my head, there is probably a few others). And I don't think Poland is considered to be an Eastern European country.
Also seems like UFC signing is based on connections sometimes -- that's why there there is a couple of Khabib's relatives on UFC roster (and I've seen some Kazakhstani or Tajikistani media wrote that Ulanbekov is a distant relative of Khabib too).
And also I think Vakhaev mentioned, not regarding UFC, just that when he was thinking that he is a free agent ACA informed him that he owns them a fight. I think I've seen also some other ex-ACA fighter said something like that -- so maybe fighters don't know/remember what is in their contracts or there is something with their contracts. And I think also some fighters/fans was saying something like that about ACA -- if you don't want to resign they take long pauses in offering fights to you or offer fights with ridiculou circumstances, or like their latest invention -- can't remmeber exactly but it's something like that bonus money or bonus for finishing your opponent will receive only fighters who signed long-term deals with them.
So, I can understand that UFC or other promotions will prefer to sign guys outside of ACA.
I also remember current situation with Duraev and DWTNCS. Duraev was making fun of Islamist victim/freedom of speech on social media and haven't fought in years yet, probably since he is a friend of Khamzat they give him a fight. Personally, I have my doubts that if he gets into the UFC he will be more successfull than Imadaev.