Users with full control access or even for that matter Site Admin access cannot edit files in SharePoint library. I have ran into this scenario multiple times in the past couple of months. It sounds crazy as this issue only happens with a selected user. Not matter if the document is Checked out before editing or not behavior is same, it just opens in read-only mode. Check out option is not available at all if the document is opened in Office client. Only available option is to save it as a different copy.
In all the cases I worked on one common thing is that user has tried to sync SharePoint library to get a local copy of files at some point. Coincidentally, issue pops up only after local sync.
That is a clear indication that something with local sync is making the documents ready only. Once i remove the SharePoint library from synced folders list, ta da! documents are back to normal – editable.
When we try to sync a SharePoint library to our local files, there is a warning message that is displayed at the bottom of OneDrive sync client folder/file selection windows which says “This site is read-only. You can’t make any changes to files or folder on this site’. If we ignore that and sync the files, all the files & folders synced will appear with a lock symbol next to the title. This is an obvious indication that these files are locked for editing.
Confusing thing here is that these files are locked for editing not just from your local OneDrive also when accessed from SharePoint. Once you sync SharePoint library to your OneDrive, they are all locked for editing. Period!
This read-only lock down does not happen for all library syncs but only for libraries with atleast one required column. Come-on now, does any library having meaningful metadata not contain a mandatory field?
I understand the reason behind this behavior as metadata is not synced locally and not available in file explorer and it gets complex beyond my thinking when there are required columns. But this locking of documents to read only mode is super confusing for end users.
This appears to be a known behavior when SharePoint libraries which contain mandatory columns are synced. Only solution I can think of is to work around this behavior.
Hope Microsoft makes it more obvious for end users why it is locking documents for editing to end user.