What do you mean "not even the un report"? Like you understand that China is a permanent member of the security Council and one of the most influential (and whiny) members of the organisation. Like would you assume Israel is innocent because the UN havnt called their actions in Palestine a genocide? And there was an independent investigation that did conclude China was commiting genocide by way of forced sterilisation and population control.

but shouldn’t Muslim-majority countries be speaking out if there was actual genocide?

Yes they should be if we assume them caring about fellow Muslims is there top, sincere priority, instead of idk securing a better trading relationship with the superpower that funds them instead of bombing them, but if that was a case they would speak out against Iran for what its doing to its citizens after the protests, or any other totalitarian leader of a Muslim country, yet they don't.

Like even if you do deny it being a genocide, there is undeniable evidence that hundreds of thousands of uyghurs have been detained in the "re-education" camps and Iran doesn't care about that either. Do you think they would be equally silent if a western country started to detain all its Muslims?

There is a good article about it here if you care to read western news articles that is

https://thediplomat.com/2018/09/irans-careful-approach-to-chinas-uyghur-crackdown/