Don't have time to dig into it right now, but over the years and versions there have been any number of buffer-flushing bugs hit SQL Server. Some of them are internal and some from Windows or even firmware/drivers (HP iLO comes to mind here from way back).

Do some web searching for such things. Also review all CU/SP after your current build for issues like you are seeing that may have been patched. There was a huge thread here on a 2012 bug that flushed randomly IIRC.