Hello,
I see you are using Excel for Android. The limitation you show in the screenshot does not show in Desktop Excel 2013, even when using merged cells (which should be avoided).
I believe that the issue lies with the Android / mobile version of Excel. Excel for mobile does not have the full functionality of desktop Excel, and it seems that this is one case where it falls behind: rendering a drop-down across all merged columns.
My suggestion is to re-arrange your spreadsheet. Unmerge the cells and widen the column with the drop-downs. Align all the entry cells below row 30 to use the same column instead of scattering the cells around different columns. If the goal is to make the spreadsheet work on Excel for Android, you may need to make some concessions to that platform and adjust the spreadsheet accordingly.
Thank you for the insight Teylyn, Unfortunately my customer will not allow us to modify their spreadsheet like that. It looks like we will have to go back to lugging around laptops for now.