The WHO has been in scandal before regarding [exorbitant spending on travel.] (https://apnews.com/3d8569ea7bd140e4ba02bc1712cd12ab)
That doesn't mean the WHO or China have clean hands, just that in an election year for the US my bias would lean towards this being about politics.
In my limited experience, people join NGOs like the WHO because they seek influence, recognition, or personal "mission" based fulfillment.
If China has been developing relationships and giving officials the above, then it absolutely could have influence far outside its financial contributions.
Does it ever occur to you that other agencies might have better relationships and trust with other countries? That people might just not respond very well to bullying money or not ?
I think if those agency have better relations with countries who ideology and goals are stark contrast with your that is corruption at least in the west eyes. If that agency ideology has change its very reasonable to leave.
What do you expect the US to do? Let very blatant misuse of their resource and personnel continue. I'm pretty sure most Americans would let their country burn before that occurred.