https://wccftech.com/kingdom-hearts-iii-remind-secret-area/

A new secret area introduced in the Kingdom Hearts III ReMIND DLC can be fully explored through a glitch.

The new area, which is the location of the DLC's secret boss, can be accessed by using Data Greeting mode and exploiting a glitch. A new video that can be watched on YouTube details how to access the area, but it includes massive spoilers so avoid watching if you have yet to play and complete the DLC and see everything it has to offer.