At the end of the Ceiros bossfight, there is a glowing square behind him that looks out of place. It is transparent and yellow, it looks to be a lighting feature that is slightly messed up. I'm fairly certain that it was supposed to fade perfectly into the background. I had the Heros mod and Cheatsheet enabled, with whichever one gives you Light Cheat, at 25%.
This is on Linux Mint, 21.3 Cinnamon (Version 6.0.4) the computer was originally a think pad, it has an intel CORE 17 vPro 7th Gen (processor i think, im not tech savvy, or mildy knowledgable, or at all knowl,edgable, im just trying to give out needed info.). The game normally lags, but ill report that in a different report. I have Heros mod and Cheatsheet, with No cheated in NPCs, Items, or anything really, I am on journey mode (No, it's just for base items duping, which I know you can do with cheatsheet or heros mod, but idk how to use them.) The Light Cheat from one of those mods is enabled, at 25%, and I revealed the map. Starlight River is normally configured, as are the others. I have not tested it again, but I can if needed.
check if the bug persists after disabling the light mod. if it doesn't persist, then that's a compatibility issue, if not, then it's an issue with something else. I don't think mod compatibility is a current goal