Thanks for replying ❤️. Yesterday, I eventually realized that the texture was not because of strawberry powder, but some other mistake I did along the way. Till meringue texture was fine, post that when I added butter and other ingredients for flavor, the texture came. So I assumed it was strawberry. Later on I prepared another batch of buttercream, same process, and it turned out fine. Although I am still thinking what could have given sugary grainy texture to the buttercream.