Physics Simulation Mode changes in play mode (but not in build)

Hi folks, like the title. weirdo thing, don’t know if I’ve accidentally changed a setting somewhere or something.
Things used to work as expected in a small test project of mine. Enter playmode, I can control my little space ship which is moved with forces.
Suddenly, when I enter play mode nothing happens. After much scratching of my head I check Project settings → physics → Simulation mode, and it is set to ‘Fixed update’. Enter play mode, and the setting changes to ‘Script’. How can this happen? When I build the project, everything works as expected with physics running in fixed update.

My first guess would be that some editor script is setting Physics.simulationMode.

Have you imported any third-party assets/utils?

Only official Unity ones so far. I’m trying to get a grip on ECS physics, is there any situation when ECS would change the setting?