Do cracks in softwood boards indicate structural problems?
Of course the splits (“checks”) will affect the performance AND strength of the wood.
Wood is a product of nature, it’s not manufactured so there will be imperfections in the wood. Wood shrinks more “across” the grain rather than than the length of the grain. So you are seeing the checks developing in each piece of wood.
Your wood is to be used for deck boards, so the material will remain outside AND will continue to shrink and split. It will not substantially affect the strength of the wood to the point that it will NEED to be replaced. However, it will become increasingly unsightly and the checks will get larger (longer and wider).