I have experienced cracks which seemed to appear from nowhere.
Like you I check the board before I cut a piece. The crack either did not exist, or was not visible and widened after the gluing.
Amazing what a little moisture can do. The joint drying could have caused small stress for this to open up.
Is this for yourself or for a customer?
If for yourself, some options :
a) Cut out the entire length of the board, e.g. 1in and glue in a strip. A narrow strip may look odd.
b) Epoxy the gap. I am with WillemJM, this looks like the crack is stable, so the epoxy should work. If you use this solution, use see if you can open the gap with e.g, screwdriver from the end, just to get better penetration. Also use masking tape around the gap so that the surrounding wood will not be sealed. Impacts whatever stain or finish you want to apply later.
c) Add a dutchman, aka bowtie insert across the crack and both boards. If you are not familiar, this is an example. In this case I added just for decoration. George Nakashima was famous for using these in his natural edge tables and chairs.
You could add at both ends and make it look like a design element.
d) Cut out a portion, e.g., 1in x the length of the crack and insert a piece. Will be structural, but would not look good.
e) If you are concerned about the crack causing some weakness, you could always drill e.g. 1/2in hole from the end and glue in a dowel for reinforcement. Would be invisible after the bread board end is attached.
Good luck with whatever solution you choose.