Could you please provide the url of the flow run where you are hitting this issue so that I can investigate for you?
The known issues I am currently working on fixing:
Files with empty content causes sending approval emails to fail to a limitation with the internal emailing service.
Approval actions don't display the proper failure message when creating approvals with attachments that are banned by the Common Data Service configuration.
@TCambs the attachments are being pulled from a SharePoint list that is populated by a PowerApp. Not sure if that makes any difference, but thought I'd point that out.
Looks like a regression, I can reproduce the problem. It's failing for a size over 8MB (for a reason unrelated to any limits). I will work on a fix along side the other known issues immediately. Thank you for bringing this to my attention.
I will update to confirm the ETA but the current estimate is that it will go out with our first deployment of the year which means it should be everywhere by Monday Jan 20th 2020.
We are now working on the functionality to attach files to Approval requests.