Libreoffice writer bypassing Linux file permissions

I have a file marked as read-only:

ls - l | grep 400.IntoTheShelterOfTheCabin.odt 
-r--r--r-- 1 stephen stephen  27K 2020-07-22 08:11 400.IntoTheShelterOfTheCabin.odt

Yet LO Writer will save changes to that same file.

• The file has been closed,
• All running instances of Writer killed.
• Writer restarted to open the file

And yet writer will save changes to the file.

I start writer with a script using this command :

/usr/bin/libreoffice --writer --norestore --nolockcheck $filename

From https://help.libreoffice.org/Common/Starting_the_Software_With_Parameters

--norestore Disables restart and file recovery after a system crash.

--nolockcheck  Disables check for remote instances using the installation.

I run the script under my UID.
I am the owner of the file.
I have full permissions on the parent directory.

Running writer without those flags makes no difference.

How is this happening? Is it expected behavior?