Hey, now that I read again, you shouldn't be able to plant anything above other plants. My bad! I'll try to check it again and see what's wrong, luckily it will be fixed by the end of the day.
Edit: So, I played my own game again, and you can place a plant of the same type above another one. However, I couldn't get the plant combination bug. Did you know exactly what happened? If you don't, don't worry, I'll try to figure out myself but it would actually be useful. Maybe I can fix it, maybe I can't (I can't actually means I could if I got the premium version). Still, it doesn't hurt to try :D
Edit 2: The bug I encountered has been fixed. It should've fixed the bug you've found as well. If it hadn't, I'd be thankful if you let me know. Again, no problem if you don't, but still. Btw I hope you like having your name saved for eternity on the description for helping me spotting a bug :D (seriously though if you want it removed, just let me know.)
Still, you've helped me much! Really thank you!