OneDrive backup files not backing up

My company recently acquired another company. We migrated their users into our tenant and had them start using our E5 licensing and OneDrive. As our OneDrive has the backup feature enabled, many users have enabled this. However, some of these new users are reporting files that they save their documents folder end up not syncing to OneDrive and when they go to find the file it is either missing or doesn't have data they have input in.

I have inspected their user account activity via Microsoft CAS and found that no actual uploads are occurring. I'm stumped as to why when they save it's not locally saving to their hard drive and awaiting a backup to OneDrive. Has anyone seen something like this before? I'm not even sure where to start troubleshooting.