Hi Kathy Minnerly,
As per your description, may I know are you open your shared Excel file document in Excel desktop client app for Mac OS? If yes, then open your shared Excel file into Excel for web(online) and do typing to see the outcomes, it will help for us to narrow down your issue whether the issue is more related to the Office 365 Excel app itself or your computer (Mac OS) environment.
Also, if you don’t mind could you please confirm does this issue happened to only specific shared Excel file?
Meanwhile, please try to operate a safe boot for Mac then the outcome. To operate the safe mode is to determine whether background programs are interfering with Office for Mac.
Disclaimer: Microsoft provides no assurances and/or warranties, implied or otherwise, and is not responsible for the information you receive from the third-party linked sites or any support related to technology.
I appreciate your understanding and stay safe!!
Best Regards
Waqas Muhammad