I believe that's the first time I have seen that reported in twenty years of using RoboHelp. Well done!
The first thing I would try is deleting the CPD file and reopening the project.
Please always start a thread with your version of RoboHelp.
See www.grainge.org for RoboHelp and Authoring information
It might be helpful if you could explain the process you are using to apply the tag to the paragraph.
For example, are you dragging and dropping? Right-clicking?
Just assume we are blind and you are having to explain it to a blind person.
I shall try this Thanks.
My version of RH is 2017.
I open a help topic. I select the content that I want to add tags to, and then apply the Author_comments tag.
After I apply a tag for example Author_comments the paragraph disappears as if I did a ctrl+x.
For some other tags that have been defined, the behavior is as expected. A colored line appears on top of the selected para.