Hi Enhance Team! @Aliysa_Enhance @Adam I've tested this for more than 10 times and now can confirm there's a bug:
I created a hosting package, assigned two server groups to the package, and selected "Allow customer to select server group placement while adding a site". Then I logged in to this test customer account and tried to add Wordpress sites. Every time I selected server group A, it still assigned websites to server group B randomly. It turns out that there is nearly a 50% chance the website was placed in B.
Please test this and fix it in future updates if you can confirm it's a bug from your side as well. Thank you very much!