One of the issues we've noticed that causes inaccurate file conflicts is when the OneDrive Sync client is stuck in a loop of syncing locally cached files on the pc. If you have Files on-demand turned on, any file that is opened or is selected to be 'keep on this device' will keep the OneDrive Sync client constantly busy, as it makes sure the latest version of that file is saved locally on the pc. So that means when the director clicks save on the excel file, there's a delay in the OneDrive Sync client updating the excel file in the cloud, in the meantime anyone else saving the file through co-authoring will have their version of the file synced before your director and hence the director will see a file conflict.
To fix this issue, open file explorer and right click on the OneDrive root folder and select 'free up space' you can do this for synced document libraries too. Free up space just removed the locally cached files and makes them cloud-omly. For a long term solution, look into setting up storage sense, comes part of Windows 10 1809 +. Set it up to clear locally cached files that have not been accessed prior to a set number of days.
You will need to apply the above fix for all members engaged in co-authoring.
HTH
