Hi Mazz,
As per informations above I think you are saying you are using “optimise iphone storage”, for iCloud photos backup option?
By doing this your original photos and videos are stored in iCloud and space-saving versions are kept on your device.
So this is only imply to photos that are safe in “photos app” that come by default with iOS that back up to iCloud .
For the second part of your question...using 3rd party app I would not have an answer for that.
You may want to connect with developer or support team of 3rd party applications that you used instead.
Here is link on optimise iphone storage: https://support.apple.com/en-us/HT204264
Good luck!