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If you have Synthetic Fulls enabled, the disk space threshold wouldn't have been triggered. That option only deletes older backup SETS, and when it does, all backups contained in the set get deleted. That option will never purge or consolidate backups in the current set, and Incrementals can't just be purged because that would break the integrity of the backup chain; they have to be consolidated. The only reason consolidation would happen automatically is if Reflect needed to do so in order to comply with your retention rules, so if you're using Incrementals Forever with Synthetic Fulls and run out of disk space on the target, jobs would just start failing until you ether got more storage or lowered your retention threshold.
You might want to check your logs to see what exactly happened and why.