I regularly work with PDFs from various authoring software applications (InDesign, Word, etc.) so am very familiar with the accessibility tagging tools in Acrobat. Recently, when trying to tag a restaurant menu in Acrobat DC (originally created in InDesign, but not exported with tags), I'm coming across a glitch with the "Reading Order" tool.
If you open the 'Touch Up Reading Order' panel located under 'Accessibility' in the Tools pane, then select an element and choose 'Background' for the content option, it will no longer allow you to tag anything else in the entire document. And of course, since there is no 'undo' function for tagging, you have to clear that page's entire structure to be able to work with this tool once again.
A workaround for this is to simply not use the background option in the 'Touch Up Reading Order' panel, and instead tag your background elements as something arbitrary, then use the tags or order panels to change the tag type. This is obviously much more cumbersome.
The glitch prevents a very useful workflow wherein you can select all elements on the page before beginning the tagging process, marking them as 'background', so when you're then finished with the tagging process there are no small elements you may have missed.
I am using the most recent version of Acrobat DC (15.020.20039) on a Windows 10 machine. I have tried the same process on another machine and consistently encounter the same glitch.