X-T3 EF+M shutter mode give soft focus

[Snapperpete]

I've been photographing polo on a bright sunny day, with my X-T3/100-400mm lens. The camera is set to 1/1000 @f5.6 auto ISO. Focus mode is AF-C zone at 9 point square. I also used single point focus with nine points. I was consistently getting soft blurry pictures which was very frustrating. I'm an experienced sports photographer and have never had a problem like this before. Then I remembered reading a recommendation to use a shutter mode of EF+M, where I had been using MS, a full manual shutter. So for the polo shots I was initially using EF+M. I switched back to MS and my hit rate increased dramatically. As anyone else experienced this issue or have any recommendations please?