It seems to be a denoise problem. The default denoise options currently offered in blender (AI, Optix and the original blender denoiser) produce jittering because they do only Spatial Denoising, meaning that they use only one frame for input, so all images are denoised separately. The main problem with this approach is that noise patterns might not be consistent or homogeneous from one frame to the next. Sometimes this kind of denoising might actually create more problems than using no denoising at all.
I wrote nice answer about video denoising software that you can use:
Denoising Animations in Cycles
You can also use a build in animation denoiser, but it only avaliable via comand line:
How to denoise animations using Blender's Temporal Denoiser?