Dear Jonah M.,
Thanks for posting back and confirming the scenario with us.
Considering that issue reproduced when we copy and paste the content from the old file, more resources need to be involved in to help you further.
We've noticed that you post this thread under "For Business" category, we suppose you may use a work/school account with M365 subscription provided by the organization.
In this situation, if using work/school account organized by the organization, we sincerely recommend you contact your Microsoft 365 administrator, follow steps in this article to create a support ticket with Online Support. The support team over there has higher permission than us, they can remotely help check the situation on your side, involve more resource to collect logs and help you do troubleshoot. This would be a more efficient way in handling this case for you.
For reference: How do I find my Microsoft 365 admin?
Otherwise, for personal account like Gmail.com, Outlook.com, Hotmail.com etc., you may contact Online Support team via Contact - Microsoft Support.
Please accept my sincere apologies for not being able to resolve the problem and for redirecting you to the related team. I regret any inconvenience this may cause. The fix to your concern can be identified through the background logs, and I suggest you raise a support ticket with Online Support team, where the related team will investigate the problem further by collecting the necessary background logs.
Once again, I apologize for the inconvenience this has caused. We appreciate your patience and understanding and thank you for your time and cooperation.
Sincerely,
Rhoda | Microsoft Community Moderator