Hello and thanks for the reply.
The camera has both depth of field and focal length animation. The DOF is not focused on the particles but on the middle mushroom. The focal length animation has a 1-degree movement for 200 frames.
The blur should be applied to the particles and not to other objects on the screen as the particles are the only ones which are moving at a speed where blur should be applied by blender.
Here are 2 different frames where 1 is rendered correctly and one has a problem, both have Motion Blur enabled.
I have tried to turn off DOF, try different focal lengths, etc etc, but nothing seems to work, the only thing I could fix this problem is by turning off motion blur.
One last thing I have to clarify. The particles are taken from an Alembic file which I have rendered beforehand. The same file is imported 2 times (because copy-pasting it from inside it, linked the animation), and I have instanced an Ico-sphere with emission to this Alembic file.
The particle systems have 600 frames. One starts at -400 frames and the other starts at 0 frames.
My video is 400 frames. With both these particles systems, I could create a perfect loop.
The second particle system which starts at 0 frames seems to have a lot more problems than the first one which starts at -400 and ends up at 200.
But from watching every frame carefully, even the First Particle system which starts at -400 has the same issue but is less visible.
I tried exporting different particle systems to see if that one I exported had problems, tried importing it from 2 different sources etc etc, but everything is fine, the particles behave very well, in the viewport everything is fine. This problem happens only when I render it.
If you want to take a look at the files you will have to do some manual work to call in the alembic files but here is a link to all the files:
Thank you very much.