Multiple Users Editing File Concurrently

Hi, hoping someone can help. Up until about 3 weeks ago if we tried to open a file that someone else had open we got an error message. Recently we had 2 people editing a document without being advised that someone else had it open and we lost a lot of important work when it was saved over by the last person to hit send. After investigating it seems that all of our documents are able to be opened by multiple users at the same time.I have checked and the "Share" function has not been used. We are on Excel for Mac in case that makes a difference. Any chance someone can help me turn off this function so only one person can work on a file. Thanks in advance.

That’s the issue, we aren’t using SharePoint. I thought there may have been a change in the settings in the last update. We don’t want to share, we need it to not let a second person in the file.

The documents are stored on a Synology Diskstation (NAS). We did upgrade this recently - we had a similar setup but just upgraded to a better version to increase speed of accessing documents etc. Could this impact the sharing of files?
In regards to your query on how we access them, we are connected to the NAS and access documents via Finder.

I put a file in Dropbox this morning and we are still able to have multiple people open the same file at the same time.