excel skips to another workbook for no apparent reason

I have a little bit to add to this.

I have the exact same problem, but I originally observed it just a little bit differently.  I noticed that Excel workbooks frequently do not respond until I click into a different workbook first and then back into the one that I am editing.  If I do click into the cell, enter data, and hit enter, I get the same results as the OP.

I eventually realized that Excel often "thinks" that I am in a different workbook because 1) If I print when this problem occurs, it shows the print preview of a different workbook than the one I am editing and prints the wrong workbook and 2) if I try to save when this happens, it saves the wrong workbook.

I am guessing that this would be the same for everyone here reporting the issue in this thread.  I don't know if this gets us anywhere since we may have already assumed that it is an issue with the active workbook, but I thought I would share since I did not see anyone else mention these two symptoms.

Unfortunately the workarounds are not really ideal in my situation, so I am really hoping for a solution.

I have the exact same issue on 2 of my user's PCs with since upgrading to Office 2016. I have tried all suggestions except for the registry one. Going to try that one to see if that helps. Thanks for keeping this going for so long!

Microsoft notes, "This method works only when you use the Excel icon to open the application. If you use File Open within the Excel application or double-click a file in Windows Explorer, the files will still open in the same instance as designed"

It's ridiculous and unacceptable to require users to employ a cumbersome workaround for an issue that has existed for over 3 years. 

How strange. Yet our symptoms have disappeared, and we do open these files from Windows Explorer. Either the explanation is wrong or outdated, or there's something else going on.

Anyway I agree that it's ridiculous. However the user community seems to have accepted the level of support that is provided. It's a pattern we see with many contemporary software products, alas.

We have been using the registry change and the basic symptom has disappeared. I don't know what you're referring to about using file explorer from within Excel. 

When researching the registry edit "solution" linked on this thread

https://support.microsoft.com/en-us/help/3165211/how-to-force-excel-to-open-in-a-new-instance-by-default

Microsoft notes, "This method works only when you use the Excel icon to open the application. If you use File Open within the Excel application or double-click a file in Windows Explorer, the files will still open in the same instance as designed"

That means this "solution" requires us to open the separate instance of Excel from the start bar and then use the File > Open > My computer > Browse, then select our internal server, THEN file down through our job sorting system (another 6 layers) to finally arrive at the folder with the file. Simply double-clicking the file within the File Explorer window I already have open for the job I'm currently working on does not let Excel open a separate instance. This is not a viable solution for me. I spend less time (albeit a marginally greater amount of frustration) simply dealing with the problem.

It's ridiculous and unacceptable to require users to employ a cumbersome workaround for an issue that has existed for over 3 years.

I have been running Excel in seperate memory spaces to try and avoid this issue for a number of years now by just doing Win+r and runnimg excel.exe /x.

Problem persists because I do need to have a number of sheets open in the same memory space for most of my work, so it is not a fix at all.

Something new however yesterday, I had a single excel sheet open and it would freeze when entering data or moving around. The only way to fix it was to change the tab on the sheet or resize the window. This looked to like the same issue but when a single sheet is open. 

For a flagship product it is very poor that MS just ignores bugs like this. Service like this only happens when you have a near monopoly status. So frustrating.