Time Machine Manual Daily Backups Being Deleted

I've recently updated to High Sierra and have noticed Time Machine, which I use manually, will only allow one backup to be kept per day. I performed three manual backups on 10th December following some app installations and now only the final backup from the day remains on the disk.

I'm backing up to a 1TB drive and there's 850GB still free, internal SSD is also only using 150GB of its capacity.

I understand that if Time Machine was set to automatic it would only keep one backup per day, but why is it deleting my manual backups when there's plenty of space? Is this normal behaviour?