Do you use iCloud for your photos? AND do you have Optimize iPhone Storage set to ON in Settings ➔ Photos?
If Yes then to optimize storage on your iPhone, low resolution photos are kept on your iPhone while the original resolution photos are maintained on iCloud. When you are first viewing your photo(s) then you are seeing the low resolution version (Blurry) and then once it is downloaded fully then you are able to see the original resolution (Clear).
If you want to maintain the original resolution photos on your iPhone at all times then you can turn OFF Optimize iPhone Storage. Note that if you do that then you will need more storage space to accommodate all your photos.
Axel F.