For over a year, i.e. while I was using LibreOffice 5, there was what I considered a bug until I figured out what is happening. Now that I’m using version 6, the bug no longer exists, but a new drawback appeared.
In the cases in question, the problem was with opening an xls file that had no password and whose “Read-only” attribute (Windows / NTFS) was not set, but in which the “Open as read-only” option was On.
In Excel, that option merely brings about a suggestion to open the file as read-only, whereas in Calc 5 it went straight to read-only mode without asking. (Not a major problem once you know that this is the source of that beheviour.)
In Calc 6, the option is totally ignored upon opening the file. Consulting the Security tab of “File | Properties” confirms that it is indeed unchecked, whereas it would have been checked in LO 5 and is still checked in Excel.
Is this a known bug in version 6?