[Bug Report] Scene covers not generating

[MrPota2]

Describe the bug
Some scenes have not gotten a cover upon first scan and some covers have been lost. When attempting to generate a new cover nothing happens.
I have been able to generate for one scene at a time by using the 'generate default thumbnail' button. This is quite tedious however.

To Reproduce
Steps to reproduce the behavior:

1. Go to 'Scenes'
2. Select scene
3. Click on the 3 dots
4. Click on generate
5. Enable 'scene covers' and 'Overwrite existing generated files'
6. Click on generate

Expected behavior
Scene cover is displayed on refresh
censored.

Stash Version: v0.20.2

Server:

• OS: Debian Bullseye
• Docker image: stashapp/stash:latest

[MrX292]

duplicate of #3657.
fixed by #3661.