Photoshop 22.0.1. Tested on three machines, one is a Mac.
How to replicate: enter/change white point or any RGB xy values, close the window. Reenter to check. Slightly different xy numbers and the white point resets to D50. It's not just a refresh bug, saved profiles reflect the displayed numbers.
Confirmed by other users too:
https://community.adobe.com/t5/photoshop/color-settings-custom-rgb-choosing-exact-custom-primaries-n...
I used/use this simple tool to derive custom linear scanner profiles based on the exact scanner color space/spectral sensitivity.