I checked today, and JOSM parses everything correctly until the file becomes malformed. It doesn’t print any error message to the console anymore and I don’t see how it will be helpful to print an error message this complicated. Most of the GPX files are created by an application, so if the file is broken your chances to fix it manually are very slim. The most likely solution would be to try to download or export the GPX again; and for this the “broken” message seems fine.
If you agree with me here, please close the ticket. Otherwise, can you explain?