Jonas-san,
The wood has ‘checked’ meaning it has dried and contracted, opening a tiny crack between growth rings of the original log/tree. This is a common defect in lumber and was not caused by rough handling - it is natural, a by-product of wood drying. I am somewhat surprised a checked piece of stock was used on such a visible location, and one that fingers touch, but I would not fret about it (groan).
I do not think the check will be a problem - both ends of the check are visible and end well inside the margins of the piece, so no chunk of wood will just split off. If the check is rough and grabs your fingers in a nasty way, a simple fix is to use a piece of beeswax or a child’s crayon of the appropriate color and rub it into the crack, building up a fat deposit of wax into and around and up and over the defect. Then carefully and slowly scrape the wax down to the level of the surrounding wood - use a popsicle stick or similar, something that is softer than the sao wood (don’t use a knife) that can scrape. Done right, this fix will completely hide the check. The sao looks like a very dense hardwood - you are unlikely to damage it further working carefully.
A luthier would do something similar but use a melted shellac stick to fill the defect - shellac is harder than wax, but also harder to work with than wax.
Knowing that the wood has checked once, you should be careful of quick or extreme changes of humidity. Going from Kanazawa with a relative humidity of ~80% to Brussels where it can be 80% in Summer down to something more like 15% indoors in a heated space in Winter. Take the same precautions you would with any wooden instrument - there are many ways to maintain a steady level moisture level around your stored shamisen.
Ganbatte,
Saburo