That’s definitely not normal behavior. Is this only happening on mobile?
You mentioned a few themes, but did you try the default as well with no custom font set? If you set a font that’s not installed or partially installed on your mobile device, that could cause unexpected issues. Other troubleshooting steps to try to see if it’s a plugin or CSS snippet causing the issue.
Also, does it happen in all notes or just one? Is there anything above that section in the note, e.g., raw HTML, an unclosed **bold section, anything like that? That’s all I can think of at the moment.