[EDIT] A quirk: I do have to hit "done" on the window before it seems to apply the change, toggling doesn't do it until I hit "done" (I just tried again to double-check and noticed this)
[EDIT 2] Nb I don't not-believe you, we could be on different OS versions (I'm on 15.1) or something else could be causing the difference.
Currently it's not autocompleting some simple algebra. But if I enter more "complex" equations ("3 / 4 =", "3 * 5 - 2 - 1 =", "tan(pi) =", etc.) then it autocompletes those. I can't figure out why it's inconsistent. And I've definitely checked and confirmed "Show inline predictive text" is disabled and I've rebooted to try and give everything a fresh start.
One thing I've noticed is that if I enter the same equation multiple times it might stop suggesting for that specific equation, so I suggest trying multiple different equations.
I've felt for some time they're overdue for an "almost nothing but bug fixes and performance improvements" major release like we got a couple times in the 20-teens :-/