Sort arrows have changed to black?

Hi MP321,

Thank you for reaching out to Microsoft Fabric Community.

In powerbi desktop, the sort arrow colour is white but in powerbi service it has changed to black colour, this could be due to a recent power bi service update that unintentionally modified the UI styling of sort indicators. It could be a temporary issue or regression that Microsoft needs to address.

To make the sort arrow visible, in the visualizations pane --> format visual, under column headers, change the background color to a different color rather than black, so that the sort arrow is visible.

If this did not resolve your issue, I would recommend to please raise a support ticket.
How to create a Fabric and Power BI Support ticket - Power BI | Microsoft Learn

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Thanks and regards,

Anjan Kumar Chippa