You have a backup which is scheduled to start 'Upon time since last backup". You change time interval in this option.
Then you notice that Next backup status turns to Not scheduled.
Next backup time is calculated according to the new setting once first backup after this changes completes.
You need to start first backup manually by clicking Run now after creating/updating backup plan schedule.
The issue has been fixed in version 9.0.
Once you have saved updated backup plan, click Run now. Start time of the next backup will be calculated and displayed upon first backup completion.