Rhythm Pattern
This instrument features multiple Rhythm Patterns. The appropriate Rhythm Pattern is determined for each Performance. This Pattern is played back using the Drum Voice assigned to Part 10 of the Performance.

Each of the MX Performances has a Rhythm Pattern associated with it that will either start automatically (AutoKeyOnStart) or you can start it manually using the Transport [PLAY] button. You can set the Rhythm Pattern to start when you touch a key using the AutoKeyOnStart feature - the transport button flashes to indicate when a Rhythm Pattern is set to start when you press a Key.

Before you assume it is stuck on or not working properly please, read through the manual… keywords: Rhythm Pattern, Song/Pattern Playing Block.
Page 16 Reference Manual
Page 59 Reference Manual

The Reference, Data List and other Manuals can be downloaded (free) from the Yamaha.com site

Link — Official Yamaha MX Manual Site

The MX has 16 Parts, an instrument sound occupies each of the slots. This is convenient for when using the MX with a DAW (Digital Audio Workstation) like Cubase AI… Think of each Part slot as a member of your band. You can play any one of the sixteen instruments, by first pressing [PART SELECT] then using the numbered buttons [1]-[16] to select the Part in question.

However, when you are in “Performance Select” mode…
Initially you can Play Part 1, you can add a second Part (Part 2) as a Layer or Split… while the Rhythm Pattern “plays” Part 10
Every factory Performance has a Rhythm Pattern associated with it - you can use it like a metronome (only it is more interesting than just the normal “click-click-click” of your standard metronome. It is quite normal for the Rhythm Pattern light to be lit when you are playing Factory Performances.

Extra Credit:
Link — Getting Started with MX