Action of hot alkaline KMnO4 on tert butyl benzene
A similar reaction with chromium oxide in alkaline solution has been documented.

The acidic chromium oxide serves the same role as alkaline KMnO4 i.e a strong oxidizing agent. As pointed out by @Waylander in the comments permanganate oxidizes double bonds even in aromatic compounds.
Reference:
$\large{•}$ Chromium Oxidations in Organic Chemistry; By G. Cainelli, G. Cardillo