Hydroxide ions don't react with un-dissociated propionic acid molecules (that's the molecule on the left).
Hydroxide ions react with the hydrogen ions on the right.
If you decrease the concentration of hydrogen ions it favours the forward reaction. Do you get it now?
Yes, they do. Hydroxide ions will react with hydronium / hydrogen ions
and with unionised propanoic acid molecules, in each case producing water and (in the latter case) the propanoate anion. Which they react with principally will depend on relative concentrations as well as collision orientation / frequency / energy, etc.
The point with a buffer is that added hydroxide either consumes hydrogen ions, causing the equilibrium to shift right to (largely) replace them, or converts propanoic acid to propanoate, thereby consuming most of the added hydroxide and resulting in minimal change in pH. By either mechanism, the addition of hydroxide causes a sudden increase in hydroxide concentration followed by a gradual decrease to a new equilibrium position where most, but not all, of the extra hydroxide has been consumed and so the pH increase is small compared to what it would have been in the absence of the buffer.
