Header does not show in posts generated with individual HTML

Hello Astra team,

I’m using the free Astra theme on my WordPress site (Gutenberg editor,
IONOS hosting). I’ve set up the header correctly via the Header Builder:

• Website Title “Wolkenhülle” is set
• Primary Menu is configured and assigned to “Primary Menu” position
• Transparent Header is OFF
• Container Layout for Single Post is set to “Content Boxed”

I have a clear diagnostic finding I’d like to share, which might help
narrow down the issue:

→ On a simple test post containing only plain text (“Test”), the header
displays correctly.

→ On the SAME post template, when I use a Custom HTML block with my own
CSS/HTML content, the header completely disappears.

This rules out a global misconfiguration (otherwise the header would be
missing in BOTH cases). The issue is specific to the combination of
“Custom HTML block + post template”.

I have tried:

• Adding position: relative and z-index: 1 to my HTML container
• Reducing the padding-top of my hero section to 40px
• Changing Container Layout settings
• Verifying the Menu is assigned to the Primary Menu position

Nothing fixes the issue. It seems like my HTML block is somehow
overlapping or pushing the header out of the viewport.

My site is currently in maintenance mode, so I can’t share a public
link, but I’m happy to share:

• Screenshots of the configuration
• The full HTML/CSS code of my block
• Comparison screenshots (with/without HTML block)

Could you help me identify what in my Custom HTML block might be
blocking the Astra header from displaying?

Thank you very much!