Updating from 2020.3.22 to .24 or .25 causing stuttering camera movement with Starter Assets

Hey so I'm having a weird issue I am struggling to track down. I am using a slightly modified version of the Starter Assets 3rd Person Controller and after updating from .22 to .24 I started noticing the camera was moving in stuttery jumps rather than smoothly. Reverting back to before the update fixes it, but changing the version using Unity Hub does not.

I have tried deleting everything in Assets and freshly reimporting the Starter Assets and I get the same issue in the example Playground scene, however the issue does not occur when starting a new project and taking the same steps (starting at .22, updating to .24, importing only Starter Assets), so there must be something in my Project Settings or something else Im not aware of that is causing the update to break it, as the code is identical between the working new project and the broken one.

I'm pretty new to Unity, so Im feeling a little at a loss here, and would really appreciate if anyone has any thoughts as to where I should look - I don't recall changing much in the settings, definitely nothing camera related.

Further details about the problem: the camera jumps around in bursts once a second or so, but the FPS remains stable, and there are no visible spikes in the Profiler, indeed running both the working and broken project I literally cannot find a single difference between them, except for the fact that running the broken one, either in play mode or in a build, has jumpy camera movement. Moving the character itself which the camera is anchored to, its just when rotating the camera with the mouse.

Thank you so much in advance for any help or pointers anyone is able to provide