Thanks for your help.
I finally solved the problem.
The reason is that the title contains special characters such as “<”, “>”, “* &”.
I use standard-notes-to-enex.py (https://github.com/tanrax/standard-notes-to-evernote-or-joplin/blob/master/standard-notes-to-enex.py) to migrate Standard Notes to Evernote, and then import to Joplin.
My Standard Notes include the above special characters, but this script don’t handle these special characters correctly.
I slightly modified the script:

title = title.replace("<","&lt;").replace(">","&gt;").replace("&","&amp;").replace("'","&apos;").replace('"',"&quot;")