Restoring previously edited source documents when opening R project

In R studio 1.1.456, when opening a R project file, previously edited source documents are not restored into editor tabs. Any ideas how I fix this? (I tried with different R project files, same outcome).
I also made sure 'Restore previously open source documents at startup' is ticked in Global options but with no effect (I even tried ticking-unticking several times).

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