In my case, there were multiple worksheets in the Excel Workbook I was opening. In my code, I was opening first worksheet #1, then worksheet #2. But when opening my workbook, it was open to #2. Switching to #1 in Excel, then saving, fixed the issue.
However, I could not replicate the bug by opening up the Excel book, switching to #2 again and saving (this should have been the initial state that was causing the bug). The code proceeded to open both worksheets without any issue.
Potentially, Excel got stuck in some sort of weird state that 'locked' the worksheets and prevented them from being activated by MATLAB (I'm using locked because I don't know a better word to use - I do not mean actual locking of the worksheet). Switching worksheets and saving may have switched Excel to a more normal state. For reference, I'm using Excel 2016 and MATLAB R2016b.