Hi @poettli, thank you for the report. Can you reproduce the issue reliably? If possible, could you please share a screen recording of the steps to reproduce?

@mjthoraval, thank you.

I see the same on Ubuntu 22.04.3 LTS with Zotero 7.0.3-beta.2

If I select the attachment directly after opening Zotero, its preview does not disappear.

As shown in the screen recording, the steps to reproduce are:
1) Select a PDF file attachment
2) Select its Parent Item
3) Go back to the PDF file attachment
You see the following message appear in the Debug Output at the time the preview disappears:
BlockingObserver: Removed observer

From what I observe:
A) Going from a parent item to another parent item, or from a child PDF to another child PDF, triggers both:
BlockingObserver: Removed observer
BlockingObserver: Added observer
B) Switching from a parent item to a child PDF, or from a child PDF to a parent item, only triggers a:
BlockingObserver: Added observer
After waiting 1 minute, you see:
BlockingObserver: Removed observer

The preview of the child PDF file attachment disappears when the Removed observer from the first switch is triggered. So you just need to wait around 1 minute (as mentioned by @poettli) after the first switch to get the Removed observer which will make the preview disappear.
The screen recording is a bit misleading, as the vanishing occurs very quickly. It was probably triggered from a previous switching before the recording.

If I wait more than one minute between steps 2 and 3, I do not see the bug anymore, which supports the analysis made above.

To be more complete, I also see the same with snapshots, with the same steps to reproduce:
https://s3.amazonaws.com/zotero.org/images/forums/u265723/n5s9zw8qnptgrx414jpu.png

Zotero 7.0.4-beta.1+576e50552 (64-bit)
Windows 10