For how many passes were these frames rendered?
Regarding the small bright “speckles” appearing in the trees - this can happen when rendering very fine details (lots of small branches + leaves) which are hard to sample. A tiny change, like a difference in the number of rendered passes or camera position, between two frames, may result in a big visual difference. The solution here would be either rendering more passes per frame (I would suggest at least 100 passes) or rendering in a larger resolution.
Regarding the overall brightness change between the frames - this could be related to the UHD Cache GI solver. Are you using the “Animation (flicker-free)” preset for the UHD Cache? More info here: https://support.chaos.com/hc/en-us/articles/4528617365649
If you need some further help, feel free to archive the scene with assets (File > Archive), or preferably a minimal version of the scene where the issue can be reproduced, and send it to us at https://support.chaos.com/hc/en-us/requests/new