Increase Mixture (Small) / Decrease Mixture (Slow) Keybinds Not Working Properly

I’m trying to assign keybinds to the Increase Mixture (Small) and Decrease Mixture (Slow) keyboard controls. Increase Mixture (Small) is assigned to Shift+Ctrl+F2 and Decrease Mixture (Slow) is assigned to Shift+Ctrl+F3. However, when I try these keybinds they move the mixture control +100% to 0%, then 0% to -100% in two steps, instead of in small amounts so that I can establish a more optimum mixture.

I tried similar bindings with the propeller control. Increase Propeller Pitch is assigned to CTRL+F3 and Decrease Propeller Pitch is assigned to CTRL+F2. These work quite well, moving the control in small increments so a good propeller pitch can be set.

If I try the Increase Mixture and Decrease Mixture keybinds (not “slow” or “small”)
with the same keys as above (Shift+Ctrl+F2 and Shift+Ctrl+F3), they move the controls by the same amounts as described. It would seem that the “slow” and “small” keybinds are not working properly in MSFS, but I might be doing something wrong. Is there a way to make the mixture controls work properly without resorting to an “Axis” keybind (which I cannot do with my setup)?