Hello @Hen Savelkoul ,
This seems more like an issue of problematic updates such as superseed updates conflicting with newer version.
First I would recommend a cleanup of the repository of one of the affected clients:
From an elevated command prompt run:
Net stop wuauserv
Net stop bits
then Delete contents of "C:\Windows\SoftwareDistribution Software Distribution" (using Windows explorer, note all the contents).
Net start wuauserv
Net start bits
After that perform a cleanup maintenance for WSUS as described here, one of the latest guides: https://learn.microsoft.com/en-us/troubleshoot/mem/configmgr/wsus-maintenance-guide
I would recommend to do a manual and visual check for superseeded or failing updates from the WSUS console:
From the WSUS Console go to:
Updates> All Updates
Set the filter to "Any except Declined"
Then right click on the "title" column head and enable the columns "File Status" and "Supersedence". This will help you detect Superseeded updates not delete and also any failed update, recommendation is to delete them.
Hope this helps with your question,
Best regards,