When a heading has subheadings under it, it is collapsible in the outline sidebar (TOC, table of contents). When you change the document so that a heading no longer has subheadings, the former-parent keeps its collapsible triangle. The icon is stale, and stuck, and remains out-of-sync with no apparent recovery (including making changes to other headings, re-opening the outline, or switching tabs).
This is surprisingly common because you typically write a document from top to bottom, and when creating the next heading, you may type one equal-sign too many, so it becomes a child of the previous heading. You can fix this immediately, but the effect is that the previous heading will remain forever styled as a parent heading, because for one brief second, it was.
- Obsidian for Mac, Version 1.11.5 (Installer 1.8.7)