Light does not spread into unloaded subchunks causing them to look dark. If only some of the blocks affected by the light source border the subchunk then only they will appear dark, blocks under the subchunk boarder will update as they should.
The darkness is client side but persists on relog. If light spread is updated the affected area will update, however it will go dark again upon reloading
Having anything in the next subchunk, like a flower, lets the light spread as it should on load. Loading the subchunk by placing a block will not fix the light spread unless the client sees a lighting update.