Hi. I have several 4K displays (one of them an old iMac with 218 ppi), so also try and set a higher font weight for readability.
I notice some strange behaviour with IBM Plex Sans (otf). This has a Regular weight (400), and a slightly heavier Text weight (450). If I select Text via GNOME Tweaks/Refine the system will actually use Regular, even though the gsettings string is “IBM Plex Sans weight=450 11”. But if I delete the Regular.otf, the system will correctly use the Text weight.
Even more strange, if I add back the Regular.otf to my local fonts, and sometime later delete it again, the UI text areas are filled with blank square boxes even though it never used the Regular.otf!
Can anyone else see if they can properly select IBM Plex Text weight as UI font when the whole family is installed?