In v15.9.* and above, when there are more than 500 rows to be exported in MUX, a background job (Export to CSV) is triggered.
When the Export to Excel icon is clicked, a banner pops up on the top of the page stating - Export is in Progress and will be available in notifications
Once the job is completed, the notification icon will have the excel ready to be downloaded however the above banner stays the same unless a refresh of the page is done.
Release : SAAS
This is a product limitation and will have to be logged as an enhancement.