Master-Master Replication: Missing PK Values
Aside from @mustaccio's link, in general, mysql will guarantee auto_increments will be unique aand monotonically increasing, but for several reasons it cannot guarantee there is no gaps.

One reason is distributelly generated ones like in your case, but it can also happen with 2 concurrent write threads on the same server- so your applications should always avoid assuming anything about concrete id growth. Use last_insert_id() and ignore gaps.