In the latest versions of Word for Microsoft 365 (e.g. ver. 2210, Build 16.0.15726.20070, Current Chanel) we noticed a similar problem when opening read-only document - in that case the Word automatically sets the Viewing mode, which means that you cannot make any changes in the document.
In previous versions of Word, when you open a read-only document, you are allowed to make changes in the document. Of course, if you want to keep the changes, you have to save the document in a new file.
Our users would like to have the previous behaviour regarding opening read-only documents but it seems there is no way to turn off the editing/reviewing/viewing feature in Word.
The strange thing is that although users have the same version of Word installed, some of them have the editing/reviewing/viewing feature turned on, while the other don't.