Adding that I did try uninstalling and reinstalling.
This is a reported problem: see Re: Why does tagging an Object as "Background" turns the Object into "Artifact" or "Sect"
Thus it remains a pain to artifact the content you want. I'm on Windows 10, so this is a DC issue, not a Mac or an XI issue. In fact, when checking work done by teammates--they tagged with XI, and I'm checking with DC--I can (so far, anyway) artifact with the TURO without issue.
A partial workaround suggested by C-student there is, instead of using the TURO Background button to artifact, select the contents in the Contents panel, right-click, and create the artifact there.
Another that sometimes works is to push the content you want artifact-ed up in the Contents panel into an artifact bin that Acrobat already created before you opened the file (in the conversion/auto-tag process).
Thank you so much! This is the answer I needed.