Update on this - I think I figured out the issue. I have one chart that always loaded with no issues, and one that wouldn't import at all. The one that wouldn't load had this:
[Editor] Bookmarks:
The one that would load had only the [Editor] part.
It seems the issue is when I create a label in the chart, then delete the label and save, it keeps the "Bookmarks:" tag in the .txt file with no data after it - this prevents the chart from being loaded until you either remove the line entirely or change it to "Bookmarks: 0". Chart loads as expected after making that change to the file itself.
So, I assume this is a bug/not intended behavior, but there's a fix for the time being