Jog Cutter mode might be turned on.

Turn the [JOG CUTTER] button off.

BPM and grid positions change suddenly sometimes when I'm using rekordbox.

You may have done this accidentally. If you spin the jog wheel while holding down the [SHIFT] button on the unit, it changes the grid.

For details, refer to [Part names and functions] in the Instruction Manual.

You can read the Instruction Manual [here].