My iPhone storage space is never released
Open the Settings App on your iPhone, go to Photos &amp; Camera.

Under the mentioned menu, you will see 2 options: Optimize iPhone Storage and Download and Keep Originals.

If Download and Keep Originals is selected, this is the cause of your issue.
Choose Optimize iPhone Storage. (Be sure that your Photos are Synced/Backed up on iCloud or on your PC or Mac - This option should not delete any photos from iCloud, but just to be safe).

Hope my answer solves your issue. I checked the settings on my iPhone 6s 64gb.