SSRS - Changing tab name when exporting to excel

You need to set the PageName of the Tablix Member (group), NOT the PageName of the Tablix itselfs.
If you got the right object, if will say "Tablix Member" (Tablix-Element in German) in the title box of the properties grid. If it's the wrong object, it will say only "table/tablix" (without member).
Also, be advised to set the sort order of the group expression, so the tabs are alphabetically sorted.

If you get the tablix instead of the tablix member, it will put the same tab name in every tab, followed by a (tabNum). That is exactly your current problem.

This solution was not working for me.

I had to add group break page.

https://www.mssqltips.com/sqlservertip/3527/export-sql-server-reporting-services-report-data-into-multiple-excel-worksheets/