Our engineers have took a look into this issue and it appears to be related to a bug on LinkedIn. The thumbnail can be edited directly on LinkedIn by an admin on the page, here's how:
To edit an Event in your LinkedIn page:
- Go to your Page super or content admin view.
- Click Events in the left menu.
- Click the Event.
- Click the Manage
button.
- Select Edit event or Manage attendees.
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- To edit: Make the necessary adjustments and click Save.
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We would also recommend reporting this issue to LinkedIn so they can take a look into it as well https://www.linkedin.com/help/linkedin/ask/puli
Updated