This problem is a result of Office Client setting a 'lock'.. that isn't a SharePoint check-out lock, and a network glitch misses clearing the lock. SharePoint usually notices within 15 or 20 minutes.. but not always.
I found a way to release this lock on-premises using PowerShell. I would think there is a PnP version that does the same on SPO.
I neglected to document where I plagiarized this from, so I cannot credit the original author
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#this script overrides a file lock that occasionally occurs when a file is opened in the client, but is closed unexpectantly.
# locks are supposed to expire after 10 minutes, but that doesn't always happen
# in this case, the file looks like it is checked in, but is locked for editing
$url = "https://WEBSITE/URL/HERE"
$fileurl = "https://WEBSITE/URL/HERE/LockedFileName.xlsx"
$web = Get-SPWeb $url
if($web -ne $null)
{
$item = $web.GetListItem($fileurl)
if($item -ne $null)
{
Write-Host "Found file " $item.Title
$item.File.CheckOutStatus
$item.ReleaseLock
#$item.File.ReleaseLock($item.File.LockId)
$userId = $item.File.LockedByUser.ID
$user = $web.AllUsers.GetById($userId)
$impSite = New-Object Microsoft.SharePoint.SPSite($web.Url, $user.UserToken);
$impWeb = $impSite.OpenWeb();
$impItem = $impWeb.GetListItem($fileurl)
$impItem.File.ReleaseLock($impItem.File.LockId)
$impWeb.Dispose()
$impSite.Dispose()
$item.File.CheckOutStatus
}
$web.Dispose()
}
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