Oh, I did some further testing and it looks like mine's one of the ones that can actually focus correctly as long as I don't move the MF slider all the way. Infinite focal length is actually at something like 2 bars below the maximum, so the slider is "too long" and just isn't calibrated properly.
Here's a GIF of what happens when it's the other way around and slider is too short though, from a Korean blog - the farthest his phone could focus to in MF was something like 20m, even though AF worked fine (the picture before the one linked is a comparison): https://m.blog.naver.com/gamja321/221837995268?view=img_6
I guess they just forgot to calibrate the MF sliders on their $1,400 phone where one of the main selling points is the camera.