@Tijke van de Ven, Thanks for posting in Q&A. Based on my researching, I find an article describe customize your logo in windows search. When Windows Search is connected to your users' work or school accounts, your organization's logo will appear in Windows Search. The ability to show your logo in the search box on the Windows taskbar is in preview. To ensure your users see the appropriate branding, verify your logo and theme in Org settings. However, please note that currently, the company logo in the taskbar search box is limited to about 80x40 with some padding and margin, so wider logos may not be legible. It may take up to 24 hours for your logo to propagate across all experiences.
You can try the above suggestion to see if it can help.
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