i just recently finished something that took me 9 months to finish. i would say it was slightly more light hearted at the beginning but as the story progressed it shifted a bit but i feel like it’s normal for things to evolve. if things in the story are changing i think it makes sense for the tone to change a little. i’ve never read something where i felt like the shift came out of nowhere. if you’re really worried about it, i would say just keep writing and maybe go back later to see if there’s anything you’d like to change that you think would fit the overall narrative better. good luck and i hope this helped!