I had the same problem, and worked meticulously for a year, convinced that it was my form. I've shot recurves and compounds -- all with finger release -- for 25-30 years, and had a lot of coaching in my younger years. While I am nowhere near perfect, I am confident that I am not doing anything bizarre in my release.
I found a way to put pressure on the string, and pull my elbow a certain way, that stopped the arrow from striking the riser, but it was not at all comfortable to shoot that way.
So, I decided to do what a lot of great bowhunters have done... I worked up a small elevated rest for my longbow. Instantly, I got lazer flight. I don't care what the "trad" culture says... I want my arrows to do what I want them to do when they leave my longbow, so I now shoot this on my bow full-time:
I should add that, prior to putting the rest on the bow, I had trouble getting any broadheads to fly straight except for vented Wensel Woodsmans. Now my others, Zwickeys, Magnus, Bodkins, 2- and 3- blades fly just fine.