iPhone battery drain with apple watch

I have an iPhone 13 mini. The battery used to last a whole day with about 8hours of screen time. Ever since getting an apple watch, the phone only lasts about 3-4 hours of on-screen time. I even noticed that it dies or loses ~30% battery in the night while I am asleep. When I check the battery graph in settings, it shows the battery draining overnight but shows nothing for both on and off-screen using the battery. This past night, I switched off the watch, and when I woke up saw very minimal power draw - how things were before getting the watch. My phone battery is at 100% health. I've had several calls and chats with apple support, even showing them the graphs, and they ran diagnostics and said nothing is wrong with the phone. I even reset the phone's settings alone once and reset the phone completely also. I've reset the watch completely more than two times now. Is this normal with the apple watch or it is a bug or something else is causing this?