I had similar issues some months ago with a pixel 9 pro. After some 'investigations', with high probability it was because of heat. Ambient temps where higher at my location, and i don't know the exact temperate, but basicly say once my phone reached 50°C it severly throttled charging speeds. Especially when the phone was in use simultanously. Charging speeds got throttled progressivly related to temp and started earlier than 50°C. I'm not sure anymore of the exact temp, but a certain temp, phone was trickle charging, and charged less power than it was using while in use.
Perhaps your issue is similar. There are battery information apps that can readout these temps. I think i used an app named 'PlusPlusBattery' or something along those lines to come to this conclusion.