Thank you for offering your support. When I was preparing the files to send to you, I found that the error disappeared! I copied all of the relevant files to a new directory. When I open my test topic file in the new directory, I no longer get the error (the error is still present in the original directory). Searching online, it appears that I am not the first person to experience this according to the following post:
So it appears that this is an Oxygen issue as opposed to a DITA issue. I'll zip the original directory and send it to you to see if you can replicate the issue.