Yes, the issue is reproducible in the sandboxed vault.
Expected result
The Outline should contain the following titles:
first title
second title
Actual result
The Outline is missing the first title, its contents are:
second title
Environment
SYSTEM INFO:
Obsidian version: v1.5.8
Installer version: v1.5.3
Operating system: Darwin Kernel Version 22.5.0: Thu Jun 8 22:22:19 PDT 2023; root:xnu-8796.121.3~7/RELEASE_ARM64_T8103 22.5.0
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Insider build toggle: off
Live preview: on
Base theme: dark
Community theme: darkplus v0.0.1
Snippets enabled: 0
Restricted mode: on
Note: I experimented further, and I found that the issue only happens if there are more than one headings in the document. In case there is only one heading, it is displayed, and the first heading only disappears once a second heading is added.
When you use that somewhat strange format for headings you end up with creating an invalid frontmatter which is why the first title disappears.
As a workaround you could therefore increase the number of dashes to avoid it being recognised as the triple dashes being used as a code fence around the frontmatter.
I believe the ticket could stay alive possibly, since it’s a situation which can occur by someone not thinking about frontmatter at all, and it can be indeed be argued that it’s a bug that Obsidian considers this as frontmatter in the first place.
I just wanted to present to you with a workaround so that you wouldn’t be hindered by this in your note taking.