I would recommend submitting a Support Case if you have not done so already for this issue. Our specialized Tableau Prep Engineers will be happy to investigate further.
Senior Technical Lead
Thanks for reaching out. I'll follow your recommendation and open a case.
It looks like you're connecting to an Excel file in AppData in a Temp folder. Do you know what application creates the file in that location? It could be that the same application is doing some cleanup since those folders are supposed to contain temporary files.
Regardless, I would recommend moving your data source out of the Temp folder in case that is the source of the problem.
The AppData folder definitely raised questions. The source file is connected to a standard, non-temp path like c:\users\rd\documents\project\proj1\data.xlsx. Seems like Prep cached a copy there in the AppData best I can tell. One thing of interest is that my Windows event logs were complaining that O365 couldn't restart. I'm wondering if there was an office update that wanted to apply and somehow killed one of the excel handles in the background causing Prep to lose sight of the temp directory. When I closed and re-opened the Flow without saving, it was able to open the original excel file without intervention. As Brandon suggested, I did open a case. I'll see if I can reproduce in the mean time.