Like most others have said, I don't believe she's trying to be mean. Honestly, my grandparents would go mental if they found out about me, so I have no intention of telling them and for the sake of not being struck off inheritance I'm gonna let them think I'm still female until they die and I get rich. I'm not sadistic, they're just evil, but rich, SOBs and it's my Dad that tells me to just keep them happy until the eejits (they're his parents) pop off and we can get their riches! Lol. I never see them now as I live in another country. At best I might phone them once a year or send a letter but they'd never really know otherwise. I already get mistaken for my brother on the phone anyway

. So first off, you're lucky that they understand and accept you as you have a really good foundation to build on here

.
I can understand where they're coming from and from someone who's not trans this may seem like a natural and easy way to avoid bullying. They don't want to see you hurt and they're trying to find a way around it until you can maybe go somewhere as a boy straight off the bat.
If it worries you, maybe just talk to them. It sounds like they're supportive of you and your transition and maybe just saying to them that acting like a girl isn't something you can do and it upsets you would clear everything up. Maybe just bring up how her words actually stung a little and you just want her to know that you are who you are and to act like a girl would be akin to asking a non trans boy to act like a girl. Ask her how she would feel if as of tomorrow she had to dress as a man and act like a man when she's out in public for just one week? That might help her understand how difficult it is for you to do the same thing and act like the gender you're not. It's nothing but an act and it feels entirely wrong, and it would drive anyone insane after a short while. It's kinda like those gay kids who go into 'Pray The Gay Away' camps and end up in therapy for the damage it's caused as they try to be something they're not. If they want a healthy grandson, he can't be required to act like the opposite gender for the sake of his sanity whether it helps with bullying or not, and frankly, any change now isn't going to make any difference because you've already come out, going back would be too obvious that you're acting to avoid bullying and only fuel the bullies more. They'll start highlighting and commenting on every girly thing you try to do or wear. You want to be yourself and not acting. Everyone who was born with the right parts get this by birth right, you're already having to ask for that right, and that's a hard enough challenge without the bullying.
If you approached it in a way where you just wanted to have a chat and discuss it further I think you could end up having a good conversation and clearing up some things. Just avoid going into it in a way that sounds like you're berating her for what she said, more just letting her know her grandson better. In fact maybe even drop in that you've already forgiven her for what she said, and you don't want her to feel bad, but you would like for her to know why just acting like a girl isn't an option. Perhaps by the time the chat is over she'll understand you even better and that can only be a good thing.
Also, it helps to remember that this stuff isn't easily understood by older generations, and even if they are aware, they usually better know about MTF than FTM so they're going to make mistakes without realising they are mistakes. When I first started kinda hinting to my parents in my teens (I'm not out to them yet) they would say things like they understood why guys would transition because guys can't exactly act girly without it being a complete gender reversal, whereas girls can get away with wearing pants and wear guys, or what are termed "unisex" these days, tshirts and it's considered normal, so why would a girl need to transition to a guy when they can essentially just wear unisex clothes and get by. So it could well be possible that your grandmother actually harvests this same view and maybe they just need to know better that you can't be unisex/androgynous, you are beyond that; you're a boy. To give another example, I used to be a home carer and I'd hear old people talk about "coloured people" or "Pakis". It was all entirely innocent; they didn't realise they were being offensive at all! So with the generation gap and changes with the time and acceptability, they sometimes just need to be pointed in the right direction on how to act or better understand.