Optimal Cutting perk almost never triggers

Well, unfortunately that is how percentages work. You only have a 10% chance for that to work, which means that it has a 90% chance to fail. And that shouldn't be read to mean that in the long run it will succeed 10% of the time. There is no universal random number monitor that will make things even out. If you are particularly unlucky, that 10% will only happen 1% of the time in your playthrough. For someone else it may happen 20% of the time. That's because the random number is rolled every time you click on the plant, so each time there is a 90% chance to fail. Your chances don't get any better because you failed 20 times in a row.

And note some plants give 2 - 3 material anyway, like Rashvine, though for plants it is much rarer than ores (some ores give 5-10 each, like Summerstone).

But overall that perk is a bad investment to me. You barely need most herbs once you've upgraded your potions and tonics. I suppose it is more helpful if you don't take the garden upgrade at your second base of operations, but still once your potions/tonics/grenades are upgraded the perk is useless. You are better off taking a perk that will help the whole game, like the codex entries. But that is just my opinion.

Your problem isn't Optimal Cutting, but bad perk criteria :P

I can hardly think of a perk more useless than that one, especially having in mind that plants infinitely respawn.