I think this is relatively normal for many to experience in one way or another....
Having windsurfed since 1978 we see the same on occasion.
I dont think it is a strain or movement, but simply because you hold the same grip for quite some time, quite static.
Doesnt matter if a boom or handles.
"Pinched", so it gets slightly sore over long time, and maybe also locally lower blood circulation.
In windsurfing even when using a harness, there is quite some grip load on both hands, and often you ride without the harness for longer times (say riding waves).
In kitesurfing you have ALL the load removed from your hands, and can hold freely with no tight grip - so no static blood pinching at all. Only unhooked, but even those dont ride like that for longer times.
What to do?
Two often used tricks:
Try holding a bit different, further out on your fingers, or tighter or looser, and change while riding.
But also, change grip from overhand to underhand quite often while you ride - this can often solve such issues by magic
And why does it, in your scenario, only hit one finger?
Also pretty normal, we are all having different "weak and strong" local points in our body, besides the fact we tend to stand and hold a tad different whether left or right foot is forward.