I believe you are running into an OOTB SharePoint issue of the file limit size. I think its 10 MBs. I am pretty sure you can use powershell in O365 to up the size. I don't have the script off hand but this is what you are running into.  If you explain why your doing a copy document we might be able to see if there is a better solution on here.

Hi Ben Stori,

Thanks for the reply
I explain, we have a custom list that has an attachment in an item of the list and that through a flow we copy those attachments to a list but a kind of library of documents in an "x" folder

So is your issue you can't find the attachments? I think I have a workflow solution to get your attachments, I am on my phone right now. I might have even created a blog post on it on the community too if you search for it. I will follow up when I am back on my PC.